CROSS JOIN又称为笛卡尔乘积，实际上是把两个表乘起来。
[实例]:
SQL CROSS JOIN will return all records where each row from the first table is combined with each row from the second table. Which also mean CROSS JOIN returns the Cartesian product of the sets of rows from the joined tables.
A CROSS JOIN can be specified in two ways: using the JOIN syntax or by listing the tables in the FROM clause separated by commas without using a WHERE clause to supply join criteria.
SQL CROSS JOIN 语法:
SELECT * FROM [TABLE 1] CROSS JOIN [TABLE 2]
OR
SELECT * FROM [TABLE 1], [TABLE 2] //重点,平时写法要注意啊
EXAMPLE :
Let’s try with 2 tables below:
Table 1: GameScores
| PlayerName | DepartmentId | Scores |
|---|---|---|
| Jason | 1 | 3000 |
| Irene | 1 | 1500 |
| Jane | 2 | 1000 |
| David | 2 | 2500 |
| Paul | 3 | 2000 |
| James | 3 | 2000 |
Table 2: Departments
| DepartmentId | DepartmentName |
|---|---|
| 1 | IT |
| 2 | Marketing |
| 3 | HR |
SQL statement :
SELECT* FROM GameScores CROSS JOIN Departments
Result:
| PlayerName | DepartmentId | Scores | DepartmentId | DepartmentName |
|---|---|---|---|---|
| Jason | 1 | 3000 | 1 | IT |
| Irene | 1 | 1500 | 1 | IT |
| Jane | 2 | 1000 | 1 | IT |
| David | 2 | 2500 | 1 | IT |
| Paul | 3 | 2000 | 1 | IT |
| James | 3 | 2000 | 1 | IT |
| Jason | 1 | 3000 | 2 | Marketing |
| Irene | 1 | 1500 | 2 | Marketing |
| Jane | 2 | 1000 | 2 | Marketing |
| David | 2 | 2500 | 2 | Marketing |
| Paul | 3 | 2000 | 2 | Marketing |
| James | 3 | 3000 | 2 | Marketing |
| Jason | 1 | 3000 | 3 | HR |
| Irene | 1 | 1500 | 3 | HR |
| Jane | 2 | 1000 | 3 | HR |
| David | 2 | 2500 | 3 | HR |
| Paul | 3 | 2000 | 3 | HR |
| James | 3 | 3000 | 3 | HR |
