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内连接 （常用）
select a.*,b.* from a inner join b on a.id = b.parent_id
结果：
|
Id |
Name |
ID |
Job |
Parent_Id |
|
1 |
张三 |
1 |
教师 |
1 |
|
2 |
李四 |
2 |
工程师 |
2 |
左连接：保留左表记录，右表被筛掉的为NULL
select a.*,b.* from a left join b on a.id = b.parent_id
结果：
|
Id |
Name |
ID |
Job |
Parent_Id |
|
1 |
张三 |
1 |
教师 |
1 |
|
2 |
李四 |
2 |
工程师 |
2 |
|
王五 |
NULL |
NULL |
NULL |
右连接：保留右表记录，左表被筛掉的为NULL
select a.*,b.* from a right join b on a.id = b.parent_id
结果：
|
Id |
Name |
ID |
Job |
Parent_Id |
|
1 |
张三 |
1 |
教师 |
1 |
|
2 |
李四 |
2 |
工程师 |
2 |
|
NULL |
NULL |
农民 |
全连接：
select a.*,b.* from a full join b on a.id = b.parent_id
结果：
| Id | Name | ID | Job | Parent_Id |
| 1 | 张三 | 1 | 教师 | 1 |
| 2 | 李四 | 2 | 工程师 | 2 |
| NULL | NULL | 农民 | ||
| 王五 | NULL | NULL | NULL |
交叉连接：表a的每条数据与表b每条数据依次匹配组合（笛卡尔积）a*b
select a.*,b.* from a cross join b on a.id = b.parent_id
结果：
| Id | Name | ID | Job | Parent_Id |
| 1 | 张三 | 1 | 教师 | 1 |
| 1 | 张三 | 2 | 工程师 | 2 |
| 1 | 张三 | 3 | 农民 | 4 |
| 2 | 李四 | 1 | 教师 | 1 |
| 2 | 李四 | 2 | 工程师 | 2 |
| 2 | 李四 | 3 | 农民 | 4 |
| 3 | 王五 | 1 | 教师 | 1 |
| 3 | 王五 | 2 | 工程师 | 2 |
| 3 | 王五 | 3 | 农民 | 4 |