SQLSubQueries(子查询)

子查询或内部查询或嵌套查询是另一个SQL查询中的查询，并嵌入在WHERE子句中。

Sub Queries- 语法

SELECT column_name [, column_name ]
FROM   table1 [, table2 ]
WHERE  column_name OPERATOR
   (SELECT column_name [, column_name ]
   FROM table1 [, table2 ]
   [WHERE])

Sub Queries- 示例

考虑具有以下记录的CUSTOMERS表-

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  35 | Ahmedabad |  2000.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  8500.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Learnfk    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

现在，让无涯教程使用SELECT语句检查以下子查询。

SQL> SELECT * 
   FROM CUSTOMERS 
   WHERE ID IN (SELECT ID 
         FROM CUSTOMERS 
         WHERE SALARY > 4500) ;

这将产生以下输出。

+----+----------+-----+---------+----------+
| ID | NAME     | AGE | ADDRESS | SALARY   |
+----+----------+-----+---------+----------+
|  4 | Chaitali |  25 | Mumbai  |  6500.00 |
|  5 | Hardik   |  27 | Bhopal  |  8500.00 |
|  7 | Learnfk    |  24 | Indore  | 10000.00 |
+----+----------+-----+---------+----------+

INSERT子查询

子查询也可以与INSERT语句一起使用， INSERT语句使用从子查询返回的数据插入另一个表。可以使用任何字符，数字功能修改子查询中的选定数据。

基本语法如下。

INSERT INTO table_name [ (column1 [, column2 ]) ]
   SELECT [ *|column1 [, column2 ]
   FROM table1 [, table2 ]
   [ WHERE VALUE OPERATOR ]

INSERT子查询- 示例

考虑具有与CUSTOMERS表相似结构的表CUSTOMERS_BKP，现在要将完整的CUSTOMERS表复制到CUSTOMERS_BKP表中，可以使用以下语法。

SQL> INSERT INTO CUSTOMERS_BKP
   SELECT * FROM CUSTOMERS 
   WHERE ID IN (SELECT ID 
   FROM CUSTOMERS) ;

UPDATE子查询

子查询可以与UPDATE语句结合使用，使用带有UPDATE语句的子查询时，可以更新表中的单列或多列。

UPDATE table
SET column_name=new_value
[ WHERE OPERATOR [ VALUE ]
   (SELECT COLUMN_NAME
   FROM TABLE_NAME)
   [ WHERE) ]

UPDATE子查询- 示例

SQL> UPDATE CUSTOMERS
   SET SALARY = SALARY * 0.25
   WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP
      WHERE AGE >= 27 );

这将影响两行，最后CUSTOMERS表将具有以下记录。

+----+----------+-----+-----------+----------+
| ID | NAME     | AGE | ADDRESS   | SALARY   |
+----+----------+-----+-----------+----------+
|  1 | Ramesh   |  35 | Ahmedabad |   125.00 |
|  2 | Khilan   |  25 | Delhi     |  1500.00 |
|  3 | kaushik  |  23 | Kota      |  2000.00 |
|  4 | Chaitali |  25 | Mumbai    |  6500.00 |
|  5 | Hardik   |  27 | Bhopal    |  2125.00 |
|  6 | Komal    |  22 | MP        |  4500.00 |
|  7 | Learnfk    |  24 | Indore    | 10000.00 |
+----+----------+-----+-----------+----------+

DELETE子查询

子查询可以与DELETE语句结合使用，就像上面提到的任何其他语句一样。

DELETE FROM TABLE_NAME
[ WHERE OPERATOR [ VALUE ]
   (SELECT COLUMN_NAME
   FROM TABLE_NAME)
   [ WHERE) ]

DELETE子查询- 示例

假设无涯教程有一个CUSTOMERS_BKP表，它是CUSTOMERS表的备份。以下示例从AGE大于或等于27的所有客户中，从CUSTOMERS表中删除记录。

SQL> DELETE FROM CUSTOMERS
   WHERE AGE IN (SELECT AGE FROM CUSTOMERS_BKP
      WHERE AGE >= 27 );

这将影响两行，最后CUSTOMERS表将具有以下记录。

+----+----------+-----+---------+----------+
| ID | NAME     | AGE | ADDRESS | SALARY   |
+----+----------+-----+---------+----------+
|  2 | Khilan   |  25 | Delhi   |  1500.00 |
|  3 | kaushik  |  23 | Kota    |  2000.00 |
|  4 | Chaitali |  25 | Mumbai  |  6500.00 |
|  6 | Komal    |  22 | MP      |  4500.00 |
|  7 | Learnfk    |  24 | Indore  | 10000.00 |
+----+----------+-----+---------+----------+