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UNION

语法说明

UNION 运算符允许您将两个或多个查询结果集合并到一个结果集中。

语法结构

SELECT column_list
UNION [DISTINCT | ALL]
SELECT column_list
UNION [DISTINCT | ALL]
SELECT column_list ...

语法说明

UNIONUNION ALL

使用 UNION 运算符组合两个或多个查询的结果集,需要满足以下条件:

  • 所有 SELECT 语句中出现的列的数量和顺序必须相同。
  • 列的数据类型必须相同或可转换。

使用 UNION ALL,则重复行(如果可用)将保留在结果中。因为 UNION ALL 不需要处理重复项。

UNIONORDER BYLIMIT

使用 ORDER BYLIMIT 子句对全部 UNION 结果进行分类或限制时,应对单个的 SELECT 语句加圆括号,并把 ORDER BYLIMIT 放到最后一个的后面。

例如:

(SELECT a FROM t1 WHERE a=10 AND B=1 ORDER BY a LIMIT 10)
UNION
(SELECT a FROM t2 WHERE a=11 AND B=2 ORDER BY a LIMIT 10);

或:

(SELECT a FROM t1 WHERE a=10 AND B=1)
UNION
(SELECT a FROM t2 WHERE a=11 AND B=2)
ORDER BY a LIMIT 10;

示例

CREATE TABLE t1 (id INT PRIMARY KEY);
CREATE TABLE t2 (id INT PRIMARY KEY);
INSERT INTO t1 VALUES (1),(2),(3);
INSERT INTO t2 VALUES (2),(3),(4);

mysql> SELECT id FROM t1 UNION SELECT id FROM t2;
+------+
| id   |
+------+
|    4 |
|    1 |
|    2 |
|    3 |
+------+

mysql> SELECT id FROM t1 UNION ALL SELECT id FROM t2;
+------+
| id   |
+------+
|    1 |
|    2 |
|    3 |
|    2 |
|    3 |
|    4 |
+------+
drop table t1;
CREATE TABLE t1 (a INT, b INT);
INSERT INTO t1 VALUES ROW(4,-2),ROW(5,9),ROW(10,1),ROW(11,2),ROW(13,5);
drop table t2;
CREATE TABLE t2 (a INT, b INT);
INSERT INTO t2 VALUES ROW(1,2),ROW(3,4),ROW(11,2),ROW(10,3),ROW(15,8);

mysql> (SELECT a FROM t1 WHERE a=10 AND B=1 ORDER BY a LIMIT 10) UNION (SELECT a FROM t2 WHERE a=11 AND B=2 ORDER BY a LIMIT 10);
+------+
| a    |
+------+
|   10 |
|   11 |
+------+