子查询
子查询或称为内部查询,嵌套查询,指的是在数据库查询的WHERE子句中嵌入查询语句,相当于临时表。一个SELECT语句的查询结果能够作为另一个语句的输入值。
子查询可以与SELECT,INSERT,UPDATE和DELETE语句一起使用。
以下是子查询必须遵守的几个规则:
- 子查询必须用括号括起来。
- 子查询在SELECT子句中只能有一个列,除非在主查询中有多列,与子查询的所选列进行比较。
- ORDER BY不能用在子查询中,虽然主查询可以使用ORDER BY。可以在子查询中使用GROUP BY,功能与ORDER BY相同。
- 子查询返回多于一行,只能与多值运算符一起使用,如IN运算符。
- BETWEEN运算符不能与子查询一起使用,但是,BETWEEN可在子查询内部使用。
SELECT语句中的子查询使用
SELECT语句在子查询返回的数据中进行查询。基本语法如下:
SELECT column_name [, column_name ]
FROM table1 [, table2 ]
WHERE column_name OPERATOR
(SELECT column_name [, column_name ]
FROM table1 [, table2 ]
[WHERE]);
示例:
创建表customer,数据内容如下。
openGauss=# SELECT * FROM customer_t1;
c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
3869 | hello | Grace | | 1000
3869 | | Grace | |
3869 | hello | | |
6985 | maps | Joes | | 2200
9976 | world | James | | 5000
4421 | Admin | Local | | 3000
(6 rows)
在SELECT中使用子查询,语句如下。
openGauss=# SELECT * FROM customer_t1 WHERE c_customer_sk IN (SELECT c_customer_sk FROM customer_t1 WHERE amount > 2500) ;
c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
9976 | world | James | | 5000
4421 | Admin | Local | | 3000
(2 rows)
INSERT语句中的子查询使用
子查询也可以与INSERT语句一起使用。INSERT语句使用子查询返回的数据插入到另一个表中。基本语法如下:
INSERT INTO table_name [ (column1 [, column2 ]) ]
SELECT [ *|column1 [, column2 ] ]
FROM table1 [, table2 ]
[ WHERE VALUE OPERATOR ]
示例:
创建表customer_bak,表结构与customer_t1一致。
openGauss=# CREATE TABLE customer_bak
(
c_customer_sk integer,
c_customer_id char(5),
c_first_name char(6),
c_last_name char(8),
Amount integer
);
CREATE TABLE
将表customer_t1中的数据插入customer_bak。
openGauss=# INSERT INTO customer_bak SELECT * FROM customer_t1 WHERE c_customer_sk IN (SELECT c_customer_sk FROM customer_t1) ;
INSERT 0 6
插入数据后的customer_bak的表如下:
openGauss=# SELECT * FROM customer_bak;
c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
3869 | hello | Grace | | 1000
3869 | | Grace | |
3869 | hello | | |
6985 | maps | Joes | | 2200
9976 | world | James | | 5000
4421 | Admin | Local | | 3000
(6 rows)
UPDATE语句中的子查询使用
通过UPDATE语句使用子查询时,表中多个列被更新。基本语法如下:
UPDATE table
SET column_name = new_value
WHERE OPERATOR [ VALUE ]
(SELECT COLUMN_NAME
FROM TABLE_NAME
[ WHERE ])
示例:
把表customer_t1中所有c_customer_sk大于4000的客户的amount更新为原来的0.50倍:
openGauss=# UPDATE customer_t1 SET amount = amount * 0.50 WHERE c_customer_sk IN (SELECT c_customer_sk FROM customer_bak WHERE c_customer_sk > 5000 );
UPDATE 2
更新影响2行,更新后表customer_t1数据如下:
openGauss=# SELECT * FROM customer_t1;
c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
3869 | hello | Grace | | 1000
3869 | | Grace | |
3869 | hello | | |
4421 | Admin | Local | | 3000
6985 | maps | Joes | | 1100
9976 | world | James | | 2500
(6 rows)
DELETE语句中的子查询使用
基本语法如下:
DELETE FROM TABLE_NAME
WHERE OPERATOR [ VALUE ]
(SELECT COLUMN_NAME
FROM TABLE_NAME
[ WHERE ])
示例:
删除表customer_t1中所有c_customer_sk大于4000的客户:
openGauss=# DELETE FROM customer_t1 WHERE c_customer_sk IN (SELECT c_customer_sk FROM customer_bak WHERE c_customer_sk > 5000 );
DELETE 2
删除影响2行,删除后的表customer_t1数据如下:
openGauss=# SELECT * FROM customer_t1;
c_customer_sk | c_customer_id | c_first_name | c_last_name | amount
---------------+---------------+--------------+-------------+--------
3869 | hello | Grace | | 1000
3869 | | Grace | |
3869 | hello | | |
4421 | Admin | Local | | 3000
(4 rows)
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