1、子查询是指在另一个查询语句中的SELECT子句。
例句：
SELECT * FROM t1 WHERE column1 = (SELECT column1 FROM t2);
其中，SELECT * FROM t1 ...称为Outer Query[外查询](或者Outer Statement),
SELECT column1 FROM t2 称为Sub Query[子查询]。
所以，子查询是指嵌套在查询内部，且必须始终出现在圆括号内。而事实上它有可能在子查询内部再嵌套子查询。
子查询可以包含多个关键字或条件，如DISTINCT,GROUP BY,ORDER BY, LIMIT,函数等。
子查询的外层查询可以是：SELECT,INSERT,UPDATE,SET或DO.
子查询可以返回标量、一行、一列或子查询。
2、使用比较运算符的子查询
=、>, <, >=, <=, <>, !=, <=>
operand comparison_operator subquery
mysql> SELECT AVG(goods_price) FROM tdb_goods; # 求平均值
+------------------+
| AVG(goods_price) |
+------------------+
| 5636.3636364 |
+------------------+
1 row in set (0.01 sec)
mysql> SELECT ROUND(AVG(goods_price), 2) FROM tdb_goods; ## 求平均值，并保留两位小数
+----------------------------+
| ROUND(AVG(goods_price), 2) |
+----------------------------+
| 5636.36 |
+----------------------------+
1 row in set (0.02 sec)
mysql> SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price
>= 5636.36; #查询
+----------+----------------------------------+-------------+
| goods_id | goods_name | goods_price |
+----------+----------------------------------+-------------+
| 3 | G150TH 15.6英寸游戏本 | 8499.000 |
| 7 | SVP13226SCB 13.3英寸触控超极本 | 7999.000 |
| 13 | iMac ME086CH/A 21.5英寸一体电脑 | 9188.000 |
| 17 | Mac Pro MD878CH/A 专业级台式电脑 | 28888.000 |
| 18 | HMZ-T3W 头戴显示设备 | 6999.000 |
| 20 | X3250 M4机架式服务器 2583i14 | 6888.000 |
| 21 | HMZ-T3W 头戴显示设备 | 6999.000 |
+----------+----------------------------------+-------------+
mysql> SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price
>= (SELECT ROUND(AVG(GOODS_PRICE), 2) FROM tdb_goods); #通过子查询实现
+----------+----------------------------------+-------------+
| goods_id | goods_name | goods_price |
+----------+----------------------------------+-------------+
| 3 | G150TH 15.6英寸游戏本 | 8499.000 |
| 7 | SVP13226SCB 13.3英寸触控超极本 | 7999.000 |
| 13 | iMac ME086CH/A 21.5英寸一体电脑 | 9188.000 |
| 17 | Mac Pro MD878CH/A 专业级台式电脑 | 28888.000 |
| 18 | HMZ-T3W 头戴显示设备 | 6999.000 |
| 20 | X3250 M4机架式服务器 2583i14 | 6888.000 |
| 21 | HMZ-T3W 头戴显示设备 | 6999.000 |
+----------+----------------------------------+-------------+
mysql> SELECT goods_price FROM tdb_goods WHERE goods_cate = '超级本';
+-------------+
| goods_price |
+-------------+
| 4999.000 |
| 4299.000 |
| 7999.000 |
+-------------+
3 rows in set (0.00 sec)
mysql> SELECT * FROM tdb_goods WHERE goods_cate = '超级本'G;
*************************** 1. row ***************************
goods_id: 5
goods_name: X240(20ALA0EYCD) 12.5英寸超极本
goods_cate: 超级本
brand_name: 联想
goods_price: 4999.000
is_show: 1
is_saleoff: 0
*************************** 2. row ***************************
goods_id: 6
goods_name: U330P 13.3英寸超极本
goods_cate: 超级本
brand_name: 联想
goods_price: 4299.000
is_show: 1
is_saleoff: 0
*************************** 3. row ***************************
goods_id: 7
goods_name: SVP13226SCB 13.3英寸触控超极本
goods_cate: 超级本
brand_name: 索尼
goods_price: 7999.000
is_show: 1
is_saleoff: 0
mysql> SELECT goods_price FROM tdb_goods WHERE goods_cate = '超级本';
+-------------+
| goods_price |
+-------------+
| 4999.000 |
| 4299.000 |
| 7999.000 |
+-------------+
mysql> SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price >
(SELECT goods_price FROM tdb_goods WHERE goods_cate = '超极本');
Empty set (0.00 sec)
mysql> SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price >
ANY (SELECT goods_price FROM tdb_goods WHERE goods_cate = '超极本');
Empty set (0.00 sec) #大于any，大于其中任何一个都可以
3、使用[NOT] IN 的子查询
ANY运算符与IN等效，
！=ALL或<>ALL运算符与NOT IN 等效
mysql> SELECT goods_id,goods_name,goods_price FROM tdb_goods WHERE goods_price !
= ALL (SELECT goods_price FROM tdb_goods WHERE goods_cate = '超极本');
其余19种记录
4、使用[NOT] EXISTS 的子查询(很少用)
如果子查询返回任何行，EXISTS将返回TRUE,否则返回FALSE.