Editing Join (SQL) (section)

==Cross join==
<code>CROSS JOIN</code> returns the [[Cartesian product]] of rows from tables in the join. In other words, it will produce rows which combine each row from the first table with each row from the second table.<ref>[http://www.sqlguides.com/sql_cross_join.php SQL CROSS JOIN]</ref>

{| class="wikitable" style="text-align:center"
|-
! Employee.LastName !! Employee.DepartmentID !! Department.DepartmentName !! Department.DepartmentID
|-
| Rafferty || 31 || Sales || 31
|-
| Jones || 33 || Sales || 31
|-
| Heisenberg || 33 || Sales || 31
|-
| Smith || 34 || Sales || 31
|-
| Robinson || 34 || Sales || 31
|-
| Williams || {{null result}} || Sales || 31
|-
| Rafferty || 31 || Engineering || 33
|-
| Jones || 33 || Engineering || 33
|-
| Heisenberg || 33 || Engineering || 33
|-
| Smith || 34 || Engineering || 33
|-
| Robinson || 34 || Engineering || 33
|-
| Williams || {{null result}} || Engineering || 33
|-
| Rafferty || 31 || Clerical || 34
|-
| Jones || 33 || Clerical || 34
|-
| Heisenberg || 33 || Clerical || 34
|-
| Smith || 34 || Clerical || 34
|-
| Robinson || 34 || Clerical || 34
|-
| Williams || {{null result}} || Clerical || 34
|-
| Rafferty || 31 || Marketing || 35
|-
| Jones || 33 || Marketing || 35
|-
| Heisenberg || 33 || Marketing || 35
|-
| Smith || 34 || Marketing || 35
|-
| Robinson || 34 || Marketing || 35
|-
| Williams || {{null result}} || Marketing || 35
|}

Example of an explicit cross join:
<syntaxhighlight lang=sql>

SELECT *
FROM employee CROSS JOIN department;
</syntaxhighlight>

Example of an implicit cross join:
<syntaxhighlight lang=sql>
SELECT *
FROM employee, department;
</syntaxhighlight>The cross join can be replaced with an inner join with an always-true condition:<syntaxhighlight lang="sql">
SELECT *
FROM employee INNER JOIN department ON 1=1;
</syntaxhighlight>

<code>CROSS JOIN</code> does not itself apply any predicate to filter rows from the joined table. The results of a <code>CROSS JOIN</code> can be filtered using a <code>WHERE</code> clause, which may then produce the equivalent of an inner join.

In the [[SQL:2011]] standard, cross joins are part of the optional F401, "Extended joined table", package.

Normal uses are for checking the server's performance.{{Why|date=May 2021}}