1.union和union all 都是实现数据库连表查询的. 但都需具备2张表的的列, 字段都一样.
2.union 实现连表查询的, 是不允许重复的, 重复值会被覆盖掉.
例如:
表一: person1
| ID | name | age |
|---|---|---|
| 1 | zhangsan | 23 |
| – | – | – |
| 2 | lisi | 24 |
| – | – | – |
| 3 | wangwu | 25 |
表二: person2
| ID | name | age |
|---|---|---|
| 1 | zhangsan | 23 |
| – | – | – |
| 2 | zhouyi | 21 |
| – | – | – |
| 3 | zhouer | 22 |
sql语句:
SELECT name FROM person1
union
SELECT name FROM person2;
查询结果:
| name |
|---|
| zhangsan |
| – |
| lisi |
| – |
| wangwu |
| – |
| zhouyi |
| – |
| zhouer |
3.union all连表查询是可以重复的.
sql语句:
SELECT name FROM person1
union
SELECT name FROM person2;
查询结果:
| name |
|---|
| zhangsan |
| – |
| lisi |
| – |
| wangwu |
| – |
| zhangsan |
| – |
| zhouyi |
| – |
| zhouer |
来源：CSDN
作者：weixin_45765705
链接：https://blog.csdn.net/weixin_45765705/article/details/104777991