# 复合查询
包含复合运算符的查询，即复合查询。所有的复合查询都具有相同的优先级，参加集合操作的各查询结果的列数、表达式的数量都必须一致，类型必须兼容。
#### UNION
常见的集合运算有：
- UNION：两个查询结果集的并集，对结果进行去重。
- UNION ALL：两个查询的并集，只将两个查询的结果合并。
 
#### UNION示例
```
-- 建表并插入数据
m_db=# CREATE TABLE test1(c11 INT, c12 VARCHAR(20));
m_db=# INSERT INTO test1 VALUES (1,'a'),(2,'b'),(4,'d');
m_db=# CREATE TABLE test2(c21 INT, c22 VARCHAR(20)); 
m_db=# INSERT INTO test2 VALUES (1,'a'),(3,'c');
```
- UNION
  ```
  m_db=# SELECT * FROM test1 UNION SELECT * FROM test2;
   c11 | c12 
  -----+-----
     4 | d
     3 | c
     1 | a
     2 | b
  (4 rows)
  ```
  
- UNION ALL
  ```
  m_db=# SELECT * FROM test1 UNION ALL SELECT * FROM test2;
   c11 | c12 
  -----+-----
     1 | a
     2 | b
     4 | d
     1 | a
     3 | c
  (5 rows)
  ```
  
 
#### EXCEPT
常见的交集运算有：
- EXCEPT：在第一个查询结果中去除两个查询结果的交集，并对结果进行去重。
- EXCEPT ALL：在第一个查询结果中去除两个查询结果的交集，不进行去重。
- EXCEPT DISTINCT：在第一个查询结果中去除两个查询结果的交集，并对结果进行去重。
 
#### EXCEPT示例
```
-- 建表并插入数据
m_db=# CREATE TABLE a(m int,n int);
m_db=# INSERT INTO a(m,n) VALUES(1,2);
m_db=# INSERT INTO a(m,n) VALUES(2,3);
m_db=# INSERT INTO a(m,n) VALUES(3,4);
m_db=# CREATE TABLE b(m int,n int);
m_db=# INSERT INTO b(m,n) VALUES(1,2);
m_db=# INSERT INTO b(m,n) VALUES(1,3);
m_db=# INSERT INTO b(m,n) VALUES(3,4);
m_db=# CREATE TABLE c(m int,n int);
m_db=# INSERT INTO c(m,n) VALUES(1,3);
m_db=# INSERT INTO c(m,n) VALUES(1,3);
m_db=# INSERT INTO c(m,n) VALUES(3,4);
```
- EXCEPT
  ```
  m_db=# SELECT * FROM a EXCEPT SELECT * FROM b;
   m | n
  ---+---
   2 | 3
  (1 row)
  m_db=# SELECT * FROM a EXCEPT SELECT * FROM c;
   m | n
  ---+---
   2 | 3
   1 | 2
  (2 rows)
  m_db=# SELECT * FROM b EXCEPT SELECT * FROM a;
   m | n
  ---+---
   1 | 3
  (1 row)
  -- 对查询结果进行去重。
  m_db=# SELECT * FROM c EXCEPT SELECT * FROM a;
   m | n
  ---+---
   1 | 3
  (1 row)
  ```
  
- EXCEPT ALL
  ```
  m_db=# SELECT * FROM a EXCEPT ALL SELECT * FROM b;
   m | n
  ---+---
   2 | 3
  (1 row)
  m_db=# SELECT * FROM a EXCEPT ALL SELECT * FROM c;
   m | n
  ---+---
   2 | 3
   1 | 2
  (2 rows)
  m_db=# SELECT * FROM b EXCEPT ALL SELECT * FROM a;
   m | n
  ---+---
   1 | 3
  (1 row)
  m_db=# SELECT * FROM c EXCEPT ALL SELECT * FROM a;
   m | n
  ---+---
   1 | 3
   1 | 3
  (2 rows)
  ```
  
- EXCEPT DISTINCT
  ```
  m_db=# SELECT * FROM a EXCEPT DISTINCT SELECT * FROM b;
   m | n
  ---+---
   2 | 3
  (1 row)
  m_db=# SELECT * FROM a EXCEPT DISTINCT SELECT * FROM c;
   m | n
  ---+---
   2 | 3
   1 | 2
  (2 rows)
  m_db=# SELECT * FROM b EXCEPT DISTINCT SELECT * FROM a;
   m | n
  ---+---
   1 | 3
  (1 row)
  -- 对查询结果进行去重。
  m_db=# SELECT * FROM c EXCEPT DISTINCT SELECT * FROM a;
   m | n
  ---+---
   1 | 3
  (1 row)
  m_db=# DROP TABLE a,b,c;
  ```