子查詢

• 語法

  • select * from 表1 where （子查詢）

• 說明

  • 外面的查詢稱為父查詢
  • 子查詢為父查詢提供查詢條件

標量子查詢

• 特點
  • 子查詢返回的值是一個

MariaDB [sel]> select * from resume where name=(select name from resume where id=3);
+----+-------+-----------+
| id | name  | skill     |
+----+-------+-----------+
|  3 | Jerry | php,mysql |
+----+-------+-----------+
# `1 row in set (0.010 sec)`
 

• 找到數學最高分

MariaDB [sel]> select * from grades where name=(select name from grades order by math desc limit 1);
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
+-------+---------+------+
# `1 row in set (0.004 sec)`

MariaDB [sel]>  select * from grades where name=(select name from grades where math=(select max(math) from grades));
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
+-------+---------+------+
# `1 row in set (0.001 sec)`
 

列子查詢

• 特點
  • 子查詢返回的結果是一列
  • 如果子查詢的結果返回多條記錄，不能使用等於，用in或not in

MariaDB [sel]> select * from grades where name in (select name from grades where math>92);
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Sunny |      93 |   96 |
| Marry |      95 |   94 |
| Tommy |      98 |   94 |
+-------+---------+------+
# `3 rows in set (0.007 sec)`
 

MariaDB [sel]> select * from grades where name not in (select name from grades where math>92);
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Jerry |      97 |   91 |
+-------+---------+------+
# `1 row in set (0.008 sec)`

MariaDB [sel]> select * from grades where name in (select name from grades where math<92);
+-------+---------+------+
| name  | chinese | math |
+-------+---------+------+
| Jerry |      97 |   91 |
+-------+---------+------+
# `1 row in set (0.000 sec)`

行子查詢

• 特點
  • 子查詢返回的結果是多個欄位組成

MariaDB [sel]> select * from grades where (sex,math) in (select sex,max(math) from grades group by sex);
+-------+------+---------+------+
| name  | sex  | chinese | math |
+-------+------+---------+------+
| Sunny | boy  |      93 |   96 |
| Marry | girl |      95 |   94 |
+-------+------+---------+------+
# `2 rows in set (0.001 sec)`

表子查詢

• 特點
  • 將子查詢的結果作為表
  • from後面跟的是資料來源，如果將子查詢當成表來看， 必須給結果集取別名

MariaDB [sel]> select * from (select * from grades order by math desc) best group by sex;
+-------+------+---------+------+
| name  | sex  | chinese | math |
+-------+------+---------+------+
| Sunny | boy  |      93 |   96 |
| Marry | girl |      95 |   94 |
+-------+------+---------+------+
# `2 rows in set (0.007 sec)`

存在子查詢

• 特點
  • 提高查詢效率

MariaDB [sel]> select * from grades where exists (select * from grades where math>90);
+-------+------+---------+------+
| name  | sex  | chinese | math |
+-------+------+---------+------+
| Sunny | boy  |      93 |   96 |
| Jerry | boy  |      97 |   91 |
| Marry | girl |      95 |   94 |
| Tommy | boy  |      98 |   94 |
+-------+------+---------+------+
# `4 rows in set (0.001 sec)`

MariaDB [sel]> select * from grades where not exists (select * from grades where math>90);
# `Empty set (0.001 sec)`