MySQL Subquery Summary

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子查询, 是指在SQL语句中内嵌了一个SELECT查询, SELECT称为内层查询, 包含SELECTSQL称为外层查询. 其按照内层查询是否依赖于外层查询, 可以分为独立子查询和相关子查询.

 

 

为了演示方便, 有测试表tb1 tb2, 数据如下:

mysql> SELECT * FROM tb1;

+------+

| col1 |

+------+

|   3 |

|   9 |

+------+

2 rows in set (0.00 sec)

 

mysql> SELECT * FROM tb2;

+------+

| col1 |

+------+

|   2 |

|   3 |

|   8 |

+------+

3 rows in set (0.00 sec)

 

独立子查询, 是指内层和外层查询没有关联,不用进行联合查询. 和其相关的有三组关键字: ANY/ IN/ SOME, ALLEXISTS.

 

ANY/ IN/ SOME的语法是:

operand comparison_operator ANY (subquery)

operand IN (subquery)

operand comparison_operator SOME (subquery)

 

a. ANY关键字的含义是, 对于在子查询返回的列中的任一值, 如果表达式结果为TRUE的话, 则返回TRUE.

 

对于如下SQL, tb1包含(3, 9), tb2包含(2, 3, 8), 表达式结果为TRUE.

mysql> SELECT col1 FROM tb1 WHERE col1> ANY (SELECT col1 FROM tb2);

+------+

| col1 |

+------+

|   3 |

|   9 |

+------+

2 rows in set (0.00 sec)

 

b. 在子查询中, = ANY的别名是IN, 下面两个SQL返回是相同的.

mysql> SELECT col1 FROM tb1WHERE col1 = ANY (SELECT col1 FROM tb2);

+------+

| col1 |

+------+

|   3 |

+------+

1 row in set (0.00 sec)

 

mysql> SELECT col1 FROM tb1WHERE col1 IN (SELECT col1 FROM tb2);

+------+

| col1 |

+------+

|   3 |

+------+

1 row in set (0.00 sec)

 

c. ANY的别名是SOME, 下面两个SQL返回是相同的.

mysql> SELECT col1 FROM tb1WHERE col1 <> ANY (SELECT col1 FROM tb2);

+------+

| col1 |

+------+

|   3 |

|   9 |

+------+

2 rows in set (0.00 sec)

 

mysql> SELECT col1 FROM tb1WHERE col1 <> SOME (SELECT col1 FROM tb2);

+------+

| col1 |

+------+

|   3 |

|   9 |

+------+

2 rows in set (0.00 sec)

 

ALL的语法是:

operand comparison_operator ALL (subquery)

 

a. ALL关键字的含义是, 对于在子查询返回的列中的所有值, 如果表达式结果为TRUE的话, 则返回TRUE.

mysql> SELECT col1 FROM tb1WHERE col1 > ALL (SELECT col1 FROM tb2);

+------+

| col1 |

+------+

|   9 |

+------+

1 row in set (0.00 sec)

 

b. <> ALL的别名是NOT IN, 下面两个SQL返回是相同的.

mysql> SELECT col1 FROM tb1 WHERE col1 <> ALL (SELECT col1 FROM tb2);

+------+

| col1 |

+------+

|    9 |

+------+

1 row in set (0.00 sec)

 

mysql> SELECT col1 FROM tb1WHERE col1 NOT IN (SELECT col1 FROM tb2);

+------+

| col1 |

+------+

|    9 |

+------+

1 row in set (0.00 sec)

 

最后一组关键字EXISTS.

 

a. EXISTS的含义是, 若子查询返回非空集, EXISTSTRUE, NOT EXISTSFALSE. 下面的SQL只是为了便于理解EXISTS,一般不这样用.

mysql>SELECT col1 FROM tb1 WHERE EXISTS (SELECT * FROMtb2);

+------+

| col1 |

+------+

|   3 |

|   9 |

+------+

  1. rows in set (0.00 sec)

 

b. 如下EXISTS子查询比较接近实际情况, 获取表tb1tb2中相同的记录. 可以看到其内层关联了外层表,这也就是下面说的相关子查询.

mysql> SELECTcol1 FROM tb1 WHERE EXISTS (SELECT * FROM tb2 WHERE tb2.col1 = tb1.col1);

+------+

| col1 |

+------+

|   3 |

+------+

1 row in set (0.00 sec)

 

 

相关子查询, 是指内层查询需要和外层查询的表相关联, 进行联合查询. 在上面已经看到了相关子查询的例子, 仔细体会下其和独立子查询语法上的差异.

 

 

另外, 如上面的SELECT col1 FROM tb1 WHERE EXISTS (SELECT * FROM tb2WHERE tb2.col1 = tb1.col1), 是和再上面的SELECT col1 FROM tb1 WHERE col1 IN (SELECT col1 FROM tb2)等价的,即可将IN的独立子查询和EXISTS的相关子查询相互改写, 那么两者有什么不同呢

 

在表tb2中加入一条记录(NULL), 其数据如下, tb1的不变:

mysql> SELECT * FROM tb2;

+------+

| col1 |

+------+

|   2 |

|   3 |

|   8 |

| NULL |

+------+

4 rows in set (0.00 sec)

 

找出在表tb1, 不在tb2中的记录, 目测结果应为9, 但返回却为空.

mysql> SELECT * FROM tb1 WHERE col1 NOTIN (SELECT col1 FROM tb2);

Empty set (0.00 sec)

 

为什么呢, 测试如下, 原来在有NULL值的情况下, NOT IN只返回NOT TRUENULL, FALSE. 就是在NULL的情况下, NOT IN永远不会返回结果.

mysql> SELECT ‘a‘ NOT IN (‘a‘, ‘b‘,NULL);

+-----------------------------+

| ‘a‘ NOT IN (‘a‘, ‘b‘, NULL) |

+-----------------------------+

|                    0 |

+-----------------------------+

1 row in set (0.00 sec)

 

mysql> SELECT ‘c‘ NOT IN (‘a‘, ‘b‘,NULL);

+-----------------------------+

| ‘c‘ NOT IN (‘a‘, ‘b‘, NULL) |

+-----------------------------+

|                 NULL |

+-----------------------------+

1 row in set (0.00 sec)

 

若想NOT IN返回结果, 要先过滤掉NULL, 这里也说明了不建议把数据存为NULL的原因.

mysql> SELECT * FROM tb1 WHERE col1 NOTIN (SELECT col1 FROM tb2 WHERE col1 IS NOT NULL);

+------+

| col1 |

+------+

|   9 |

+------+

1 row in set (0.00 sec)

 

使用EXISTS来写的话, 由于其是相关子查询,不用特殊考虑NULL的情况.

mysql> SELECT * FROM tb1 WHERE NOTEXISTS (SELECT col1 FROM tb2 WHERE tb2.col1 = tb1.col1);

+------+

| col1 |

+------+

|    9 |

+------+

1 row in set (0.00 sec)

 

 

对于子查询的性能优化, 以及改写为JOIN, 稍后会进行整理, 感兴趣可关注订阅号”数据库最佳实践”(DBBestPractice).

 

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