MySQL多表查询之纵向合并

纵向合并查询又称为联合查询，它是用union关键字把多条select语句的查询结果合并为一个结果集。
纵向合并的前提是被合并的结果集的字段数量、顺序和数据类型必须完全一致。字段名不一样的情况下，会将第一个结果集的字段名作为合并后的虚拟结果集的字段名。

select <字段1>[,<字段2>,…] from <表名1>
union[ all] 
select <字段1>[,<字段2>,…] from <表名2>;

create table t1(key1 char,v1 int);
create table t2(key2 char,v2 int);

insert into t1 values('a',1),('a',2),('b',3),('c',4),('a',13);
insert into t2 values('b',10),('b',11),('a',12),('a',13),('e',14);

• union去重

mysql> select * from t1
    -> union
    -> select * from t2;
+------+------+
| key1 | v1   |
+------+------+
| a    |    1 |
| a    |    2 |
| b    |    3 |
| c    |    4 |
| a    |   13 |
| b    |   10 |
| b    |   11 |
| a    |   12 |
| e    |   14 |
+------+------+

• union all不去重

mysql> select * from t1
    -> union all
    -> select * from t2;
+------+------+
| key1 | v1   |
+------+------+
| a    |    1 |
| a    |    2 |
| b    |    3 |
| c    |    4 |
| a    |   13 |
| b    |   10 |
| b    |   11 |
| a    |   12 |
| a    |   13 |
| e    |   14 |
+------+------+