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12.2.8.1. JOIN Syntax

MySQL supports the following JOIN syntaxes for the table_references part of SELECT statements and multiple-table DELETE and UPDATE statements: 

table_references:
    table_reference [, table_reference] ...

table_reference:
    table_factor
  | join_table

table_factor:
    tbl_name [[AS] alias] [index_hint_list]
  | table_subquery [AS] alias
  | ( table_references )
  | { OJ table_reference LEFT OUTER JOIN table_reference
        ON conditional_expr }

join_table:
    table_reference [INNER | CROSS] JOIN table_factor [join_condition]
  | table_reference STRAIGHT_JOIN table_factor
  | table_reference STRAIGHT_JOIN table_factor ON conditional_expr
  | table_reference {LEFT|RIGHT} [OUTER] JOIN table_reference join_condition
  | table_reference NATURAL [{LEFT|RIGHT} [OUTER]] JOIN table_factor

join_condition:
    ON conditional_expr
  | USING (column_list)

index_hint_list:
    index_hint [, index_hint] ...

index_hint:
    USE {INDEX|KEY}
      [{FOR {JOIN|ORDER BY|GROUP BY}] ([index_list])
  | IGNORE {INDEX|KEY}
      [{FOR {JOIN|ORDER BY|GROUP BY}] (index_list)
  | FORCE {INDEX|KEY}
      [{FOR {JOIN|ORDER BY|GROUP BY}] (index_list)

index_list:
    index_name [, index_name] ...

A table reference is also known as a join expression.

The syntax of table_factor is extended in comparison with the SQL Standard. The latter accepts only table_reference, not a list of them inside a pair of parentheses.

This is a conservative extension if we consider each comma in a list of table_reference items as equivalent to an inner join. For example: 

SELECT * FROM t1 LEFT JOIN (t2, t3, t4)
                 ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)

is equivalent to: 

SELECT * FROM t1 LEFT JOIN (t2 CROSS JOIN t3 CROSS JOIN t4)
                 ON (t2.a=t1.a AND t3.b=t1.b AND t4.c=t1.c)

In MySQL, CROSS JOIN is a syntactic equivalent to INNER JOIN (they can replace each other). In standard SQL, they are not equivalent. INNER JOIN is used with an ON clause, CROSS JOIN is used otherwise.

In general, parentheses can be ignored in join expressions containing only inner join operations. MySQL also supports nested joins (see Section 7.2.11, “Nested Join Optimization”).

Index hints can be specified to affect how the MySQL optimizer makes use of indexes. For more information, see Section 12.2.8.2, “Index Hint Syntax”.

The following list describes general factors to take into account when writing joins.

  • A table reference can be aliased using tbl_name AS alias_name or tbl_name alias_name: 

    SELECT t1.name, t2.salary
  FROM employee AS t1 INNER JOIN info AS t2 ON t1.name = t2.name;

SELECT t1.name, t2.salary
  FROM employee t1 INNER JOIN info t2 ON t1.name = t2.name;

  • A table_subquery is also known as a subquery in the FROM clause. Such subqueries must include an alias to give the subquery result a table name. A trivial example follows; see also Section 12.2.9.8, “Subqueries in the FROM clause”. 

    SELECT * FROM (SELECT 1, 2, 3) AS t1;

  • INNER JOIN and , (comma) are semantically equivalent in the absence of a join condition: both produce a Cartesian product between the specified tables (that is, each and every row in the first table is joined to each and every row in the second table).

    However, the precedence of the comma operator is less than of INNER JOIN, CROSS JOIN, LEFT JOIN, and so on. If you mix comma joins with the other join types when there is a join condition, an error of the form Unknown column 'col_name' in 'on clause' may occur. Information about dealing with this problem is given later in this section.

  • The conditional_expr used with ON is any conditional expression of the form that can be used in a WHERE clause. Generally, you should use the ON clause for conditions that specify how to join tables, and the WHERE clause to restrict which rows you want in the result set.

  • If there is no matching row for the right table in the ON or USING part in a LEFT JOIN, a row with all columns set to NULL is used for the right table. You can use this fact to find rows in a table that have no counterpart in another table: 

    SELECT left_tbl.*
  FROM left_tbl LEFT JOIN right_tbl ON left_tbl.id = right_tbl.id
  WHERE right_tbl.id IS NULL;

    This example finds all rows in left_tbl with an id value that is not present in right_tbl (that is, all rows in left_tbl with no corresponding row in right_tbl). This assumes that right_tbl.id is declared NOT NULL. See Section 7.2.9, “LEFT JOIN and RIGHT JOIN Optimization”.

  • The USING(column_list) clause names a list of columns that must exist in both tables. If tables a and b both contain columns c1, c2, and c3, the following join compares corresponding columns from the two tables: 

    a LEFT JOIN b USING (c1,c2,c3)

  • The NATURAL [LEFT] JOIN of two tables is defined to be semantically equivalent to an INNER JOIN or a LEFT JOIN with a USING clause that names all columns that exist in both tables.

  • RIGHT JOIN works analogously to LEFT JOIN. To keep code portable across databases, it is recommended that you use LEFT JOIN instead of RIGHT JOIN.

  • The { OJ ... LEFT OUTER JOIN ...} syntax shown in the join syntax description exists only for compatibility with ODBC. The curly braces in the syntax should be written literally; they are not metasyntax as used elsewhere in syntax descriptions. 

    SELECT left_tbl.*
    FROM { OJ left_tbl LEFT OUTER JOIN right_tbl ON left_tbl.id = right_tbl.id }
    WHERE right_tbl.id IS NULL;

    As of MySQL 5.1.24, you can use other types of joins within { OJ ... }, such as INNER JOIN or RIGHT OUTER JOIN. This helps with compatibility with some third-party applications, but is not official ODBC syntax.

  • STRAIGHT_JOIN is similar to JOIN, except that the left table is always read before the right table. This can be used for those (few) cases for which the join optimizer puts the tables in the wrong order.

Some join examples: 

SELECT * FROM table1, table2;

SELECT * FROM table1 INNER JOIN table2 ON table1.id=table2.id;

SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id;

SELECT * FROM table1 LEFT JOIN table2 USING (id);

SELECT * FROM table1 LEFT JOIN table2 ON table1.id=table2.id
  LEFT JOIN table3 ON table2.id=table3.id;

Join Processing Changes in MySQL 5.0.12

Note

Natural joins and joins with USING, including outer join variants, are processed according to the SQL:2003 standard. The goal was to align the syntax and semantics of MySQL with respect to NATURAL JOIN and JOIN ... USING according to SQL:2003. However, these changes in join processing can result in different output columns for some joins. Also, some queries that appeared to work correctly in older versions (prior to 5.0.12) must be rewritten to comply with the standard.

These changes have five main aspects:

  • The way that MySQL determines the result columns of NATURAL or USING join operations (and thus the result of the entire FROM clause).

  • Expansion of SELECT * and SELECT tbl_name.* into a list of selected columns.

  • Resolution of column names in NATURAL or USING joins.

  • Transformation of NATURAL or USING joins into JOIN ... ON.

  • Resolution of column names in the ON condition of a JOIN ... ON.

The following list provides more detail about several effects of current join processing versus join processing in older versions. The term “previously” means “prior to MySQL 5.0.12.”

  • The columns of a NATURAL join or a USING join may be different from previously. Specifically, redundant output columns no longer appear, and the order of columns for SELECT * expansion may be different from before.

    Consider this set of statements: 

    CREATE TABLE t1 (i INT, j INT);
CREATE TABLE t2 (k INT, j INT);
INSERT INTO t1 VALUES(1,1);
INSERT INTO t2 VALUES(1,1);
SELECT * FROM t1 NATURAL JOIN t2;
SELECT * FROM t1 JOIN t2 USING (j);

    Previously, the statements produced this output: 

    +------+------+------+------+
| i    | j    | k    | j    |
+------+------+------+------+
|    1 |    1 |    1 |    1 |
+------+------+------+------+
+------+------+------+------+
| i    | j    | k    | j    |
+------+------+------+------+
|    1 |    1 |    1 |    1 |
+------+------+------+------+

    In the first SELECT statement, column j appears in both tables and thus becomes a join column, so, according to standard SQL, it should appear only once in the output, not twice. Similarly, in the second SELECT statement, column j is named in the USING clause and should appear only once in the output, not twice. But in both cases, the redundant column is not eliminated. Also, the order of the columns is not correct according to standard SQL.

    Now the statements produce this output: 

    +------+------+------+
| j    | i    | k    |
+------+------+------+
|    1 |    1 |    1 |
+------+------+------+
+------+------+------+
| j    | i    | k    |
+------+------+------+
|    1 |    1 |    1 |
+------+------+------+

    The redundant column is eliminated and the column order is correct according to standard SQL:

    • First, coalesced common columns of the two joined tables, in the order in which they occur in the first table

    • Second, columns unique to the first table, in order in which they occur in that table

    • Third, columns unique to the second table, in order in which they occur in that table

    The single result column that replaces two common columns is defined via the coalesce operation. That is, for two t1.a and t2.a the resulting single join column a is defined as a = COALESCE(t1.a, t2.a), where: 

    COALESCE(x, y) = (CASE WHEN V1 IS NOT NULL THEN V1 ELSE V2 END)

    If the join operation is any other join, the result columns of the join consists of the concatenation of all columns of the joined tables. This is the same as previously.

    A consequence of the definition of coalesced columns is that, for outer joins, the coalesced column contains the value of the non-NULL column if one of the two columns is always NULL. If neither or both columns are NULL, both common columns have the same value, so it doesn't matter which one is chosen as the value of the coalesced column. A simple way to interpret this is to consider that a coalesced column of an outer join is represented by the common column of the inner table of a JOIN. Suppose that the tables t1(a,b) and t2(a,c) have the following contents: 

    t1    t2
----  ----
1 x   2 z
2 y   3 w

    Then: 

    mysql> SELECT * FROM t1 NATURAL LEFT JOIN t2;
+------+------+------+
| a    | b    | c    |
+------+------+------+
|    1 | x    | NULL |
|    2 | y    | z    |
+------+------+------+

    Here column a contains the values of t1.a. 

    mysql> SELECT * FROM t1 NATURAL RIGHT JOIN t2;
+------+------+------+
| a    | c    | b    |
+------+------+------+
|    2 | z    | y    |
|    3 | w    | NULL |
+------+------+------+

    Here column a contains the values of t2.a.

    Compare these results to the otherwise equivalent queries with JOIN ... ON: 

    mysql> SELECT * FROM t1 LEFT JOIN t2 ON (t1.a = t2.a);
+------+------+------+------+
| a    | b    | a    | c    |
+------+------+------+------+
|    1 | x    | NULL | NULL |
|    2 | y    |    2 | z    |
+------+------+------+------+

    mysql> SELECT * FROM t1 RIGHT JOIN t2 ON (t1.a = t2.a);
+------+------+------+------+
| a    | b    | a    | c    |
+------+------+------+------+
|    2 | y    |    2 | z    |
| NULL | NULL |    3 | w    |
+------+------+------+------+

  • Previously, a USING clause could be rewritten as an ON clause that compares corresponding columns. For example, the following two clauses were semantically identical: 

    a LEFT JOIN b USING (c1,c2,c3)
a LEFT JOIN b ON a.c1=b.c1 AND a.c2=b.c2 AND a.c3=b.c3

    Now the two clauses no longer are quite the same:

    • With respect to determining which rows satisfy the join condition, both joins remain semantically identical.

    • With respect to determining which columns to display for SELECT * expansion, the two joins are not semantically identical. The USING join selects the coalesced value of corresponding columns, whereas the ON join selects all columns from all tables. For the preceding USING join, SELECT * selects these values: 

      COALESCE(a.c1,b.c1), COALESCE(a.c2,b.c2), COALESCE(a.c3,b.c3)

      For the ON join, SELECT * selects these values: 

      a.c1, a.c2, a.c3, b.c1, b.c2, b.c3

      With an inner join, COALESCE(a.c1,b.c1) is the same as either a.c1 or b.c1 because both columns will have the same value. With an outer join (such as LEFT JOIN), one of the two columns can be NULL. That column will be omitted from the result.

  • The evaluation of multi-way natural joins differs in a very important way that affects the result of NATURAL or USING joins and that can require query rewriting. Suppose that you have three tables t1(a,b), t2(c,b), and t3(a,c) that each have one row: t1(1,2), t2(10,2), and t3(7,10). Suppose also that you have this NATURAL JOIN on the three tables: 

    SELECT ... FROM t1 NATURAL JOIN t2 NATURAL JOIN t3;

    Previously, the left operand of the second join was considered to be t2, whereas it should be the nested join (t1 NATURAL JOIN t2). As a result, the columns of t3 are checked for common columns only in t2, and, if t3 has common columns with t1, these columns are not used as equi-join columns. Thus, previously, the preceding query was transformed to the following equi-join: 

    SELECT ... FROM t1, t2, t3
  WHERE t1.b = t2.b AND t2.c = t3.c;

    That join is missing one more equi-join predicate (t1.a = t3.a). As a result, it produces one row, not the empty result that it should. The correct equivalent query is this: 

    SELECT ... FROM t1, t2, t3
  WHERE t1.b = t2.b AND t2.c = t3.c AND t1.a = t3.a;

    If you require the same query result in current versions of MySQL as in older versions, rewrite the natural join as the first equi-join.

  • Previously, the comma operator (,) and JOIN both had the same precedence, so the join expression t1, t2 JOIN t3 was interpreted as ((t1, t2) JOIN t3). Now JOIN has higher precedence, so the expression is interpreted as (t1, (t2 JOIN t3)). This change affects statements that use an ON clause, because that clause can refer only to columns in the operands of the join, and the change in precedence changes interpretation of what those operands are.

    Example: 

    CREATE TABLE t1 (i1 INT, j1 INT);
CREATE TABLE t2 (i2 INT, j2 INT);
CREATE TABLE t3 (i3 INT, j3 INT);
INSERT INTO t1 VALUES(1,1);
INSERT INTO t2 VALUES(1,1);
INSERT INTO t3 VALUES(1,1);
SELECT * FROM t1, t2 JOIN t3 ON (t1.i1 = t3.i3);

    Previously, the SELECT was legal due to the implicit grouping of t1,t2 as (t1,t2). Now the JOIN takes precedence, so the operands for the ON clause are t2 and t3. Because t1.i1 is not a column in either of the operands, the result is an Unknown column 't1.i1' in 'on clause' error. To allow the join to be processed, group the first two tables explicitly with parentheses so that the operands for the ON clause are (t1,t2) and t3: 

    SELECT * FROM (t1, t2) JOIN t3 ON (t1.i1 = t3.i3);

    Alternatively, avoid the use of the comma operator and use JOIN instead: 

    SELECT * FROM t1 JOIN t2 JOIN t3 ON (t1.i1 = t3.i3);

    This change also applies to statements that mix the comma operator with INNER JOIN, CROSS JOIN, LEFT JOIN, and RIGHT JOIN, all of which now have higher precedence than the comma operator.

  • Previously, the ON clause could refer to columns in tables named to its right. Now an ON clause can refer only to its operands.

    Example: 

    CREATE TABLE t1 (i1 INT);
CREATE TABLE t2 (i2 INT);
CREATE TABLE t3 (i3 INT);
SELECT * FROM t1 JOIN t2 ON (i1 = i3) JOIN t3;

    Previously, the SELECT statement was legal. Now the statement fails with an Unknown column 'i3' in 'on clause' error because i3 is a column in t3, which is not an operand of the ON clause. The statement should be rewritten as follows: 

    SELECT * FROM t1 JOIN t2 JOIN t3 ON (i1 = i3);

  • Resolution of column names in NATURAL or USING joins is different than previously. For column names that are outside the FROM clause, MySQL now handles a superset of the queries compared to previously. That is, in cases when MySQL formerly issued an error that some column is ambiguous, the query now is handled correctly. This is due to the fact that MySQL now treats the common columns of NATURAL or USING joins as a single column, so when a query refers to such columns, the query compiler does not consider them as ambiguous.

    Example: 

    SELECT * FROM t1 NATURAL JOIN t2 WHERE b > 1;

    Previously, this query would produce an error ERROR 1052 (23000): Column 'b' in where clause is ambiguous. Now the query produces the correct result: 

    +------+------+------+
| b    | c    | y    |
+------+------+------+
|    4 |    2 |    3 |
+------+------+------+

    One extension of MySQL compared to the SQL:2003 standard is that MySQL allows you to qualify the common (coalesced) columns of NATURAL or USING joins (just as previously), while the standard disallows that.


User Comments

Posted by Scott Atkins on January 23 2003 6:43am[Delete] [Edit]

Tip time:

(Background: This database is used to keep track of scores for students in my classes.)

So in this case, I have three tables, one has student's "codename" (as posting their real name on the web is a no-no) and an index (there is more data in this table, but this is all you really need to know.) Then there's a table with the assignments, containing the assignment name, and an index for each assignment. Finally, there is a scores table, which has for each paper I get turned in, a student_id (releated to the student index) an act_id (related to the assignments index) and a score.

It looked something like this:
students table:

+----+---------------+
| id | codename      |
+----+---------------+
|  1 | Budy          |
+----+---------------+

assignments table:
+--------+------------+
| act_id | name       |
+--------+------------+
|      1 | Activity 1 |
|      2 | Activity 2 |
+--------+------------+

scores table:
+------------+--------+-------+
| student_id | act_id | score |
+------------+--------+-------+
|          1 |      1 |    10 |
|          1 |      2 |    10 |
+------------+--------+-------+

Now the problem was, I wanted to have the assignments listed across the top, and the scores next to the names. Something like this:
+---------------+------------+------------+-------+
| codename      | Activity 1 | Activity 2 | Total |
+---------------+------------+------------+-------+
|          budy |         10 |         10 |    20 |
+---------------+------------+------------+-------+

So here's how the sql statement ended up:
SELECT names.codename,
s1.score AS "Score1", s1.comment AS "Comments1",
s2.score AS "Score2", s2.comment AS "Comments2",
SUM(st.score) AS "Total"
FROM students names
LEFT JOIN scores s1 ON s1.act_id=1 AND names.id=s1.student_id
LEFT JOIN scores s2 ON s2.act_id=2 AND names.id=s2.student_id
LEFT JOIN scores st ON names.id=st.student_id
WHERE names.codename <> ''
GROUP BY names.codename
ORDER BY names.codename;

As you can see, for each activity, I need to add another left join, but it looks exactly like the last one, thus it is easy to build through a program like php. I hope this helps someone out.

Posted by Thomas Mayer on April 21 2003 6:58pm[Delete] [Edit]

I use left joins to generate sums on one table using different conditions:
t1 to make sure that ALL grouped records are shown
t(n+1) for use per condition
and as mentioned above, the JOIN condition must be used as well for the primary key AND for the condtion per sum!

Here is an example:

drop table if exists testtable;
create table testtable
(mykey int not null,
mygroup int,
cond int,
value int,
primary key (mykey));

insert into testtable
values (1, 1, 1, 5), (2, 1, 1, 6), (3, 1, 2, 3), (4, 2, 2, 4), (5, 3, 3, 5);

-- returns nothing
select t1.mygroup, sum(t2.value) as cond_1, sum(t3.value) as cond_2, sum(t4.value) as cond_3
from testtable t1
left join testtable t2 on t1.mykey=t2.mykey
left join testtable t3 on t1.mykey=t3.mykey
left join testtable t4 on t1.mykey=t4.mykey
where t2.cond=1
and t3.cond=2
and t4.cond=3
group by 1
order by 1;

-- returns correct sums
select t1.mygroup, sum(t2.value) as cond_1, sum(t3.value) as cond_2, sum(t4.value) as cond_3
from testtable t1
left join testtable t2 on t1.mykey=t2.mykey and t2.cond=1
left join testtable t3 on t1.mykey=t3.mykey and t3.cond=2
left join testtable t4 on t1.mykey=t4.mykey and t4.cond=3
group by 1
order by 1;

mygroup | cond_1 | cond_2 | cond_3
1 | 11 | 3 | 0
2 | 0 | 4 | 0
3 | 0 | 0 | 5

Posted by joerg schaber on June 11 2003 2:24am[Delete] [Edit]

I also think that the missing feature of FULL OUTER JOIN is a real drawback to MySQL. However, from MySQL 4 on you can use a workaround using the UNION construct. E.g. at
http://www.oreillynet.com/pub/a/network/2002/04/23/fulljoin.html

Posted by Y G on October 27 2003 3:52am[Delete] [Edit]

Below is an example of how to left-join multiple tables independently.

SELECT ...
FROM table 1
LEFT JOIN table 2 on (table1.id = table2.id)
LEFT JOIN table 3 on (table1.id2 = table3.id2)
LEFT JOIN table 4 on (table1.id3 = table4.id3)

Posted by name withheld on November 13 2003 1:33pm[Delete] [Edit]

Martin,
your comment & others helped emensely!
Here's a left-join select that also has a regular join to another table.
I want to get all Plans (& associated SubType info),
but also see which plans user 13 is signed up
for, but only if the expire_date hasn't passed.
This select will show all Plan & SubType info,
but user-info only if the user is signed up,
and the expire-date hasn't passed.

SELECT
*
FROM mt_SubTypes, mt_Plans as t1
LEFT JOIN mt_UserPlans as t2
on (t1.id_plan = t2.id_plan
and t2.expire_date > '2003-11-12'
and t2.id_user = 13)
WHERE
t1.id_subType = mt_SubTypes.id_subType;

Posted by Cory McHugh on December 22 2003 2:54pm[Delete] [Edit]

This is an example of using a left to get lookup values from a table twice. The reason that an outer join was used instead of an inner join, was that in this case, there may be values that are null inside of the degree table.

SELECT d.degDegId, m1.majDescription AS major_1, m2.majDescription AS major_2
FROM degree AS d
LEFT OUTER JOIN major AS m1
ON d.degMajor1 = m1.majMajId
LEFT OUTER JOIN major AS m2
ON d.degMajor2 = m2.majMajId

Posted by Joonas Kekoni on August 19 2004 6:37am[Delete] [Edit]

The oracle outter join syntax:
select a1.a,a1.b,a2.a,a2.b from a1,a2 where a1.a=a2.b(+);

Is expressed like:
select a1.a,a1.b,a2.a,a2.b from a1 left join a2 on a2.b=a1.a where 1=1;

NOTE: a2 is the optional table:

(PLEASE NOTE you must NOT specify a2 in from line(!!).
The 1=1 is optional,but I added it here so you could see where to add other conditions).


full example:
drop table a1; drop table a2;
create table a1 (a integer,b integer);
create table a2 (a integer,b integer);
insert into a1 (a,b) values (1,2);
insert into a1 (a,b) values (3,5);
insert into a2 (a,b) values (2,1);
insert into a2 (a,b) values (3,5);
select a1.a,a1.b,a2.a,a2.b from a1 left join a2 on a2.b=a1.a where 1=1;
drop table a1; drop table a2;

PS.
I prefer the Oracle syntax and would like to see it supported by MySql too.

Posted by Fred Mitchell on December 11 2004 1:47pm[Delete] [Edit]

Let's say you are doing a LEFT JOIN with a table that shares a column name in common with another table, and that you are selecting for instances where the join is missing, that is IS NULL.

Normally, the common column name is "wiped out" by the null record, but here is a workaround for it: You simply alias that common column name in the select. For instance,

CREATE TABLE t1 (INT id NOT NULL, ....);
CREATE TABLE t2 (INT id NOT NULL, ....);
...
SELECT * FROM t1 LEFT JOIN t2 ON t1.id = t2.id
WHERE t2.id IS NULL;

would result in the column 'id' being null on each selected row. Instead, you can do:

SELECT *, t1.id AS id FROM t1 LEFT JOIN t2 ON t1.id = t2.id
WHERE t2.id IS NULL;

And now the 'id' column will be preserved since the alias is evaluated *after* the LEFT JOIN.

Posted by [name withheld] on December 14 2004 12:08pm[Delete] [Edit]

Note that table_reference in the above grammar may also be a "anonymous table" resulting from a join, as shown below. I don't know if this is intended or by occasion, but for me it works (MySQL 4.0.22):

SELECT products.id
FROM products
LEFT OUTER JOIN (
author2products
INNER JOIN author ON author.id = author2products.authorid
) ON products.id = author2products.productsid

This enables you to OUTER JOIN with the results of an INNER JOIN within one query.

Posted by Ben Griffin on June 30 2005 11:23am[Delete] [Edit]

Scenario: A group of companies have a set of publications. Any company in the group can subscribe to any number of publications.
Requirement: Create a query to generate a list of companies, showing the count of publications and subscriptions for each company.

create table company ( cid int not null, cref char(64) not null,primary key (cid));
create table publication ( pid int not null, pcid int not null, pref char(64) not null,primary key (pid));
create table subscription ( scid int not null,spid int not null,primary key (scid,spid));

insert into company values (1,'A Corp');
insert into company values (2,'B Corp');
insert into company values (3,'C Corp');
insert into company values (4,'D Corp');
insert into company values (5,'E Corp');

insert into publication values (10,1,'A News');
insert into publication values (11,1,'A Digest');
insert into publication values (12,1,'A Review');
insert into publication values (20,2,'B News');
insert into publication values (51,5,'E Digest');
insert into publication values (52,5,'E Review');

insert into subscription values (1,10);
insert into subscription values (1,11);
insert into subscription values (1,20);
insert into subscription values (1,51);
insert into subscription values (1,52);
insert into subscription values (2,10);
insert into subscription values (4,10);
insert into subscription values (4,52);

select cref,count(distinct pid) as pubs,count(distinct spid) as subs from company left join publication on cid=pcid left join subscription on cid=scid group by cid order by cref;

+--------+------+------+
| cref   | pubs | subs |
+--------+------+------+
| A Corp |    3 |    5 |
| B Corp |    1 |    1 |
| C Corp |    0 |    0 |
| D Corp |    0 |    2 |
| E Corp |    2 |    0 |
+--------+------+------+

Posted by Mike Nix on August 24 2005 4:11am[Delete] [Edit]

This checks for an event that should happen yearly, depending on other factors - it could be applied to checking for any event that should occur at regular intervals - ie monthly or weekly - with adjustments to the code for selecting max_calves.

This is a fairly complex example, with lots of joins, aliases and grouping... Its aim is to extract a list of cows which have not calved every year in a given period. It accounts for cows which have not been in the database for the entire period, are not currently on our property (locations.local==1), or which are too young to have calves for the entire period (they can't calve before 2 years old).
There are many actions that signal entry/exit of an animal - eg birth/buy/sell/death. many (or none) of them may be recorded in the history table. As a fallback, animal.yob is the year of birth of the animal.
Cows and their calves are all stored as "animals" in the animals table (the mob field distinguishes between them).

The query returns the number of calves they did have in the time frame (num_calves) and the number of calves they should have had (max_calves).

Tables: (columns not relevant to query have been removed)
create table history (id int(10), date datetime, action int(2) not null, info text, primary key(id,date));
create table actions (id int(2) auto_increment primary key, name char(20), type char(20));
create table mob_types (id int(2) primary key, name char(20));
create table locations (id int(2) auto_increment primary key, name char(20), alive tinyint(1), local tinyint(1));
create table animals (id int(10) auto_increment primary key, yob int(4), tag1 char(10), mother int(10), mob int(2), location int(2));

Foreign keys:
animals.id <- history.id
animals.location -> locations.id
animals.mob -> mobs.id
mobs.type -> mob_types.id
history.action -> actions.id

The SQL to make it happen:
The original formatting is easier to read, but the leading spaces are not preserved here (the site won't let me use html to get it either)
$xxx are the input variables:
$start_year, $end_year: date range to check (inclusive)
$all_animals: 0= alive and on property, 1= everything
$nomiss: 0= those that missed a year, 1=those that didn't miss
$allow_miss: number of misses considered acceptable.


select a1.*,aquire.date,disposal.date,count(a2.id) num_calves,
(IF(IsNull(disposal.date),$end_year,
IF(DATE_FORMAT(disposal.date,'%Y') < $end_year,DATE_FORMAT(disposal.date,'%Y'),$end_year) -
IF(DATE_FORMAT(aquire.date,'%Y')>$start_year,
IF(DATE_FORMAT(aquire.date,'%Y')>(a1.yob+2),DATE_FORMAT(aquire.date,'%Y'),a1.yob+2),
IF(a1.yob>($start_year-2),a1.yob+2,$start_year)
)
+1) as max_calves
from animals as a1
left join animals as a2
on a1.id=a2.mother
and a2.yob>=$start_year
left join history as aquire
on a1.id=aquire.id
and aquire.action in (select id from actions where type='aquire')
left join history as disposal
on a1.id=disposal.id
and disposal.action in (select id from actions where type='disposal')
where a1.mob in (select id from mobs where mobtype in (select id from mob_types where name='Cow'))
and (($all_animals>0)
or a1.location in (select id from locations where alive=1 and local=1))
group by a1.id
having IF($nomiss>0, num_calves>=(max_calves-$allow_miss), num_calves<(max_calves-$allow_miss))
order by a1.location,cast(a1.tag1 as unsigned),a1.tag1;

Speed: with about 6000 records in the animals table, and appropriate indexes, this returns in under half a second on a modern machine (versus several seconds without indexes)

I still need to add support for twins (two events in same year should be counted as one), but I'm not sure how to do that yet (any suggestions?).

Posted by Normann Aa. Nielsen on November 24 2005 10:03am[Delete] [Edit]

If you are running older versions of Bugzilla you should be aware that you'll have to do some modification yourself, because of the change in the JOIN-syntax. For a nice to-do, see http://www.chuckcaplan.com/blog/archives/2005/10/bugzilla_and_my.html

That really helped me!

Posted by Odimar Tomazeli on January 19 2006 7:13pm[Delete] [Edit]

Here a example join 3 tables like that

s_g_a_t
............|
............+-> s_c
.....................|
.....................+-> s_t_i

SELECT distinct sc.TType, si.Id
FROM s_g_a_t st LEFT JOIN s_c sc ON(st.GA = sc.GA and sc.id_app = 'XXX' and sc.Select = 1) INNER JOIN s_t_i si ON (sc.TType = si.DL)
Order by si.Id

Posted by [name withheld] on June 15 2006 8:36pm[Delete] [Edit]

This is an example for joining tables which are related by 2 fields.

Squeme:

tblA(idA, idB1, idB2)
tblB(idB, Name)
relations: tblA.idB1->tblB.IdB
tblB.idB2->tblB.IdB

and we want to obtain all records of tblA with its related tblB.Name

The sql statment could be like this:

SELECT
tblA.IdA,
tblB.Name AS Name1,
tblB_1.Name AS Name2
FROM
tblA INNER JOIN
tblB tblB_1 ON tblA.IdB1 = tblB_1.IdB
INNER JOIN tblB
ON tblA.IdB2 = tblB.IdB

(tested in MySql v4.0.24)

Posted by barbarina on September 12 2006 2:17pm[Delete] [Edit]

You can emulate FULL OUTER JOIN using UNION (from MySQL 4.0.0 on):


with two tables t1, t2:

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id


with three tables t1, t2, t3:

SELECT * FROM t1
LEFT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
LEFT JOIN t3 ON t2.id = t3.id
UNION
SELECT * FROM t1
RIGHT JOIN t2 ON t1.id = t2.id
RIGHT JOIN t3 ON t2.id = t3.id

Posted by Tobias Riemenschneider on September 26 2006 12:54pm[Delete] [Edit]

The result of a full outer join betwee tables A and B includes:
- rows from the result of the inner join
- rows from A that don't have corresponding rows in B
- rows from B that don't have corresponding rows in A

Formally, the corresponding SQL statement looks like this:

SELECT *
FROM A JOIN B ON A.id = B.id
UNION ALL
SELECT *
FROM A LEFT JOIN B ON A.id = B.id
WHERE B.id IS NULL
UNION ALL
SELECT *
FROM A RIGHT JOIN B ON A.id = B.id
WHERE A.id IS NULL

Due to the fact that the first union represents a left outer join, the statement can be simplified:

SELECT *
FROM A LEFT JOIN B ON A.id = B.id
UNION ALL
SELECT *
FROM A RIGHT JOIN B ON A.id = B.id
WHERE A.id IS NULL

Posted by [name withheld] on March 16 2007 3:51pm[Delete] [Edit]

The example posted on September 12 2006 about how to emulate FULL OUTER JOIN using UNION has a subtle problem. Assume you have 3 tables, each with one single colum "id" and 4 rows.
t0 contains 1,3,5,7,
t1 contains 2,3,6,7,
t2 contains 4,5,6,7
(ti contains j iff 2^i xor j = 1).

The suggested solution:

SELECT * FROM t0 LEFT JOIN t1 ON t0.id = t1.id LEFT JOIN t2 ON t1.id = t2.id UNION
SELECT * FROM t0 RIGHT JOIN t1 ON t0.id = t1.id LEFT JOIN t2 ON t1.id = t2.id UNION
SELECT * FROM t0 RIGHT JOIN t1 ON t0.id = t1.id RIGHT JOIN t2 ON t1.id = t2.id

produces

id id id
1 NULL NULL
3 3 NULL
5 NULL NULL
7 7 7
NULL 2 NULL
NULL 6 6
NULL NULL 4
NULL NULL 5

where "5" appears 2 times. To get the correct result, use only LEFT JOIN, start once with each table, and name the columns:

SELECT t0.id as id0, t1.id as id1, t2.id as id2 FROM t0 LEFT JOIN t1 USING(id) LEFT JOIN t2 USING(id) UNION
SELECT t0.id as id0, t1.id as id1, t2.id as id2 FROM t1 LEFT JOIN t0 USING(id) LEFT JOIN t2 USING(id) UNION
SELECT t0.id as id0, t1.id as id1, t2.id as id2 FROM t2 LEFT JOIN t1 USING(id) LEFT JOIN t0 USING(id)
ORDER BY COALESCE(id0,id1,id2)

The ORDER BY is optional. The length of the query grows as the square of the number of tables, which is quite bad. FULL OUTER JOIN would be really welcome.

Posted by Balaji Devarajan on March 18 2007 7:50pm[Delete] [Edit]

I found this union LEFT JOIN AND RIGHT JOIN, display all the columns from the two tables very usefull for me...

mysql> select * from NAME;

+------+------+
| ID   | NAME |
+------+------+
|    1 | bala |
|    2 | renu |
+------+------+
mysql> select * from DESCR;
+------+-------+
| ID   | DESCR |
+------+-------+
|    2 | BBB   |
|    3 | CCC   |
+------+-------+
mysql> SELECT t1.ID,t1.NAME,t2.DESCR FROM NAME t1 LEFT JOIN DESCR t2 ON (t1.ID = t2.ID) UNION SELECT t2.ID,t1.NAME,t2.DESCR FROM NAME t1 RIGHT JOIN DESCR t
2 ON (t1.ID = t2.ID);
+------+------+-------+
| ID   | NAME | DESCR |
+------+------+-------+
|    1 | bala | NULL  |
|    2 | renu | BBB   |
|    3 | NULL | CCC   |
+------+------+-------+

Posted by Ted Conn on December 21 2007 9:38am[Delete] [Edit]

Use group_concat to coalesce fields that are otherwise impossible when using an inner join.

For example:

SELECT T1.a,T1.b,T2.a,GROUP_CONCAT(T2.b ORDER BY T2.b DESC SEPARATOR '|') as d FROM T1 INNER JOIN T2 ON T1.a = T2.b GROUP BY T1.a

When this query might otherwise duplicate rows, adding the group_concat coalesces them as a blob which when printed returns all the values concatenated by the defined separator string!

Posted by Wiebe Cazemier on March 25 2008 7:18pm[Delete] [Edit]

For those who don't fully understand the concept of joins, I wrote an article which might help.

http://www.halfgaar.net/sql-joins-are-easy

Posted by Geoffrey De Smet on July 29 2008 8:03am[Delete] [Edit]

Faking a full outer join through unions doesn't work when you need to do grouping to calculate totals.

For example:

select a.x, a.y, sum(a.price), sum(b.price)
from A a full join B b on a.x = b.x and a.y = b.y
where ...
group by a.x, a.y
order by a.x, a.y

A full join is needed because there are (x,y) combinations that exist only in A or only in B.
A group by is needed because in B the (x,y) combination isn't unique.

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