MySQL中常用操作--子查询

　　子查询就是SELECT 查询是另一个SELECT的附属,  此时, MySQL从最内层的查询开始, 再向外向上查询, 接着最外层的父查询被执行, 它的结果也指定给父查询.

• 　　带IN的子查询:
  只有子查询返回的结果列中包含一个值时, 比较运算符才适用. 假如一个子查询的返回结果集是值的列表, 这时比较运算符就必须用关键字IN代替
  IN运算符可以检测结果集中是否存在某个特定的值, 如果检测成功就执行外部的查询.

  mysql> select * from stu
      -> ;
  +----+----------+-------+
  | id | username | score |
  +----+----------+-------+
  |  1 | liub     |    95 |
  |  2 | xiaoming |   100 |
  |  3 | xiaohong |    80 |
  |  4 | 周星星   |     0 |
  |  5 | 凌凌漆   |   100 |
  |  6 | 达叔     |    90 |
  |  7 | 浩南     |    60 |
  |  8 | 山鸡     |    50 |
  |  9 | 孙猴子   |    85 |
  | 10 | 李四     |    55 |
  +----+----------+-------+
  10 rows in set (0.00 sec)
  
  mysql> SELECT * FROM scoreLevel;
  +----+-------+
  | id | score |
  +----+-------+
  |  1 |    90 |
  |  2 |    80 |
  |  3 |    70 |
  +----+-------+
  3 rows in set (0.00 sec)
  
  mysql> SELECT * FROM stu WHERE score IN (SELECT score FROM scoreLevel);
  +----+----------+-------+
  | id | username | score |
  +----+----------+-------+
  |  3 | xiaohong |    80 |
  |  6 | 达叔     |    90 |
  +----+----------+-------+
  2 rows in set (0.00 sec)

  NOT IN :

  mysql> SELECT * FROM stu WHERE score NOT IN (SELECT score FROM scoreLevel);
  +----+----------+-------+
  | id | username | score |
  +----+----------+-------+
  |  1 | liub     |    95 |
  |  2 | xiaoming |   100 |
  |  4 | 周星星   |     0 |
  |  5 | 凌凌漆   |   100 |
  |  7 | 浩南     |    60 |
  |  8 | 山鸡     |    50 |
  |  9 | 孙猴子   |    85 |
  | 10 | 李四     |    55 |
  +----+----------+-------+
  8 rows in set (0.00 sec)

   

• 带比较运算符的子查询:
  =, !=, >, >=, <, <=......
  

   

   SELECT * FROM stu WHERE score>=(SELECT score FROM scoreLevel WHERE id=1);

  mysql> SELECT * FROM stu WHERE score>=(SELECT score FROM scoreLevel WHERE id=1);
  +----+----------+-------+
  | id | username | score |
  +----+----------+-------+
  |  1 | liub     |    95 |
  |  2 | xiaoming |   100 |
  |  5 | 凌凌漆   |   100 |
  |  6 | 达叔     |    90 |
  +----+----------+-------+
  4 rows in set (0.00 sec)

   

• 带关键字EXISTS 的子查询:
  使用关键字EXISTS查询时, 内层查询语句不返回查询的记录, 而是返回一个真假值. 如果内层的查询语句查询到满足条件的语句的记录, 就返回一个真值(true), 否则返回一个假值(false).当返回的值为true时, 外层查询语句将进行查询, 当返回结果为false时, 外层查询语句不进行查询或者查询不出任何记录.
  
  SELECT * FROM stu WHERE EXISTS (SELECT score FROM scoreLevel WHERE id=2);
  
  

  mysql> SELECT * FROM stu WHERE EXISTS (SELECT score FROM scoreLevel WHERE id=10);
  Empty set (0.00 sec)
  
  mysql> SELECT * FROM stu WHERE EXISTS (SELECT score FROM scoreLevel WHERE id=2);
  +----+----------+-------+
  | id | username | score |
  +----+----------+-------+
  |  1 | liub     |    95 |
  |  2 | xiaoming |   100 |
  |  3 | xiaohong |    80 |
  |  4 | 周星星   |     0 |
  |  5 | 凌凌漆   |   100 |
  |  6 | 达叔     |    90 |
  |  7 | 浩南     |    60 |
  |  8 | 山鸡     |    50 |
  |  9 | 孙猴子   |    85 |
  | 10 | 李四     |    55 |
  +----+----------+-------+
  10 rows in set (0.00 sec)

   

• 带ANY, SOME, ALL 的查询:
  
  

  mysql> SELECT * FROM stu WHERE score>=ANY(SELECT score FROM scoreLevel);--表示在 stu 中 >= scoreLevel 中的最小值(70)//SOME 和 ANY效果一样 
  +----+----------+-------+
  | id | username | score |
  +----+----------+-------+
  |  1 | liub     |    95 |
  |  2 | xiaoming |   100 |
  |  3 | xiaohong |    80 |
  |  5 | 凌凌漆   |   100 |
  |  6 | 达叔     |    90 |
  |  9 | 孙猴子   |    85 |
  +----+----------+-------+
  6 rows in set (0.00 sec)

mysql> SELECT * FROM stu WHERE score>=ALL(SELECT score FROM scoreLevel);--表示在 stu 中 >= scoreLevel 中的最大值(90)
+----+----------+-------+
| id | username | score |
+----+----------+-------+
|  1 | liub     |    95 |
|  2 | xiaoming |   100 |
|  5 | 凌凌漆   |   100 |
|  6 | 达叔     |    90 |
+----+----------+-------+
4 rows in set (0.00 sec)

CREATE .......SELECT .......:

mysql> CREATE table excellentStudent(
    -> id TINYINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
    -> username VARCHAR(100) NOT NULL,
    -> score TINYINT UNSIGNED NOT NULL
    -> )ENGINE=INNODB CHARSET=UTF8 SELECT * FROM stu WHERE score>=ALL(SELECT score FROM scoreLevel);
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0


mysql> SELECT * FROM excellentstudent;
+----+----------+-------+
| id | username | score |
+----+----------+-------+
|  1 | liub     |    95 |
|  2 | xiaoming |   100 |
|  5 | 凌凌漆   |   100 |
|  6 | 达叔     |    90 |
+----+----------+-------+
4 rows in set (0.00 sec)

INSERT.......SELECT.........:

mysql> INSERT excellentStudent(username) SELECT username FROM user;
Query OK, 7 rows affected, 1 warning (0.01 sec)
Records: 7  Duplicates: 0  Warnings: 1

mysql> SELECT * FROM excellentstudent;
+----+----------+-------+
| id | username | score |
+----+----------+-------+
|  1 | liub     |    95 |
|  2 | xiaoming |   100 |
|  5 | 凌凌漆   |   100 |
|  6 | 达叔     |    90 |
|  7 | a        |     0 |
|  8 | b        |     0 |
|  9 | c        |     0 |
| 10 | d        |     0 |
| 11 | e        |     0 |
| 12 | f        |     0 |
| 13 | g        |     0 |
+----+----------+-------+
11 rows in set (0.00 sec)

去掉字段的重复值:

SELECT DISTINCT(字段名) FROM 表名;

创建一个与table_name2一样的表结构的table_name1(创建好的table_name1有结构, 但是内容为空.):

CREATE TABLE table_name1 LIKE table_name2;

• 合并查询结果

　　　　合并查询结果是将多个SELECT语句的查询结果合并到一起, 使用关键字 UNION 和 UNION ALL.其中, UNION 是将多个表中的记录去重后合并到一起, 而 UNION ALL 只是简单的合并操作.　　

　　UNION:
　　　　SELECT 字段名称,... FROM tbl_name1
　　　　UNION
　　　　SELECT 字段名称... FROM tbl_name2;
　　UNION ALL:
　　　　SELECT 字段名称,... FROM tbl_name1
　　　　UNION ALL
　　　　SELECT 字段名称... FROM tbl_name2;