delete语句触发索引

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mob64ca14089531 2024-11-25 22:47:43
文章标签 delete语句触发索引数据库 mysql 表名逻辑运算符 文章分类 数据仓库大数据
mysql delete操作

语法:delete from 表名 where.

mysql> select * from user;
+----+--------+---------------------+
| id | name   | birthday            |
+----+--------+---------------------+
|  1 | lisi   | 2000-08-09 00:00:00 |
|  2 | saluya | 2010-01-11 00:00:00 |
|  3 | lucy   | 1993-07-26 00:00:00 |
+----+--------+---------------------+
3 rows in set (0.00 sec)

mysql> delete from user where id = 3;
Query OK, 1 row affected (0.01 sec)

mysql> select * from user;
+----+--------+---------------------+
| id | name   | birthday            |
+----+--------+---------------------+
|  1 | lisi   | 2000-08-09 00:00:00 |
|  2 | saluya | 2010-01-11 00:00:00 |
+----+--------+---------------------+
2 rows in set (0.00 sec)

注意:如果后面不跟where条件过滤，所有的行都会收到影响。

mysql select操作

语法：select 列1,列2.... from 表名 where expr.

#查询所有的数据,数据量过大会使表崩。
mysql> select * from user;
+----+--------+---------------------+
| id | name   | birthday            |
+----+--------+---------------------+
|  1 | lisi   | 2000-08-09 00:00:00 |
|  2 | saluya | 2010-01-11 00:00:00 |
+----+--------+---------------------+
2 rows in set (0.00 sec)

#增加where条件过滤，只查id=2的行。
mysql> select * from user where id=2;
+----+--------+---------------------+
| id | name   | birthday            |
+----+--------+---------------------+
|  2 | saluya | 2010-01-11 00:00:00 |
+----+--------+---------------------+
1 row in set (0.00 sec)

#查询id>=2的行。
mysql> select * from user where id>=2;
+----+--------+---------------------+
| id | name   | birthday            |
+----+--------+---------------------+
|  2 | saluya | 2010-01-11 00:00:00 |
|  3 | lili   | 2009-09-01 00:00:00 |
+----+--------+---------------------+
2 rows in set (0.00 sec)

#固定查询只查看哪几列。
mysql> select id,name from user where id>=2;
+----+--------+
| id | name   |
+----+--------+
|  2 | saluya |
|  3 | lili   |
+----+--------+
2 rows in set (0.00 sec)

select查询模型:
列是变量，where是表达式，值为真假.

#注意:1为true，0为fales，直接写ture也是可以的。
mysql> select id,name from user where 1;
+----+--------+
| id | name   |
+----+--------+
|  1 | lisi   |
|  2 | saluya |
|  3 | lili   |
+----+--------+
3 rows in set (0.00 sec)

#注意:变量为假一行也打印不出来.
mysql> select id,name from user where 0;
Empty set (0.00 sec)

#注意:where条件写布尔型也是可以匹配到的.
mysql> select id,name from user where true;
+----+--------+
| id | name   |
+----+--------+
|  1 | lisi   |
|  2 | saluya |
|  3 | lili   |
+----+--------+
3 rows in set (0.00 sec)

比较运算符：
    等于        =
    不等于        != 或者 <>
    小于        <
    小于等于        <=
    大于(等于)        >(=)
    大于等于        >=
    在某集合内    in    (等于in(a,b,...,n)中的任意一个值都行
    在某个范围内    between    (在between a and b 之间的值都可以，允许等于边界值)
        逻辑运算符：
    逻辑非        NOT 或 !
    逻辑或        OR 或 ||
    逻辑与        AND 或 &&
        模糊查询：
    像        like    
    通配符：
    通配任意字符    %    （like(诺基亚%)）
    通配单个字符    _    （like(诺基亚N__)）

#变量可以做‘+’，‘-’运算，在mysql数据库种也一样,但是要针对可以做运算的列.
mysql> select id+1 from user  where 1;
+------+
| id+1 |
+------+
|    2 |
|    3 |
|    4 |
+------+

错误案例：
如果不能做加减运算的列，结果如下：
mysql> select id,name+1 from user  where 1;
+----+--------+
| id | name+1 |
+----+--------+
|  1 |      1 |
|  2 |      1 |
|  3 |      1 |
+----+--------+
3 rows in set, 3 warnings (0.00 sec)

#mysql列与列之间可以做“+”，“-”法运算，称之为广义投影.
mysql> select * from student;
+-----+--------+------+------+
| SNO | SNAME  | AGE  | SEX  |
+-----+--------+------+------+
| 1   | 李强   |   23 | 男   |
| 2   | 刘丽   |   22 | 女   |
| 5   | 张友   |   22 | 男   |
+-----+--------+------+------+
3 rows in set (0.00 sec)

mysql> select SNAME,SEX,AGE-SNO from student;
+--------+------+---------+
| SNAME  | SEX  | AGE-SNO |
+--------+------+---------+
| 李强   | 男   |      22 |
| 刘丽   | 女   |      20 |
| 张友   | 男   |      17 |
+--------+------+---------+
3 rows in set (0.00 sec)

mysql> select * from student;
+-----+--------+------+------+
| SNO | SNAME  | AGE  | SEX  |
+-----+--------+------+------+
| 1   | 李强   |   23 | 男   |
| 2   | 刘丽   |   22 | 女   |
| 5   | 张友   |   22 | 男   |
+-----+--------+------+------+
3 rows in set (0.00 sec)

#查找student表中，所有年龄不等于!=22和<>22的学生名字，两种写法都行.
mysql> select SNAME from student where AGE != 22;
+--------+
| SNAME  |
+--------+
| 李强   |
+--------+
1 row in set (0.00 sec)

mysql> select SNAME from student where AGE <> 22;
+--------+
| SNAME  |
+--------+
| 李强   |
+--------+
1 row in set (0.00 sec)

#查询年级大于22岁的学生名字.
mysql> select SNAME from student where AGE > 22;
+--------+
| SNAME  |
+--------+
| 李强   |
+--------+
1 row in set (0.00 sec)

#查询年龄等于22或者等于23的学生名字,"or"用法.
mysql> select SNAME,AGE from student where AGE=23 or AGE= 22;
+--------+------+
| SNAME  | AGE  |
+--------+------+
| 李强   |   23 |
| 刘丽   |   22 |
| 张友   |   22 |
+--------+------+
3 rows in set (0.00 sec)

#查询年龄在22,23的学生名字,"in"用法，在某几个值之间.
mysql> select SNAME,AGE from student where AGE in (22,23);
+--------+------+
| SNAME  | AGE  |
+--------+------+
| 李强   |   23 |
| 刘丽   |   22 |
| 张友   |   22 |
+--------+------+
3 rows in set (0.00 sec)

#查询年龄在22,23的学生名字,"between"用法，取值范围之间.
mysql> select SNAME,AGE from student where AGE between 22 and 23;
+--------+------+
| SNAME  | AGE  |
+--------+------+
| 李强   |   23 |
| 刘丽   |   22 |
| 张友   |   22 |
+--------+------+
3 rows in set (0.00 sec)

mysql> select SNAME,AGE from student where AGE between 0 and 23;
+--------+------+
| SNAME  | AGE  |
+--------+------+
| 李强   |   23 |
| 刘丽   |   22 |
| 张友   |   22 |
+--------+------+
3 rows in set (0.00 sec)

#找出年龄不等于23也不等于23的学生,“not in”用法.
mysql> select SNAME,AGE from student where AGE != 22 and AGE !=23;
Empty set (0.00 sec)

mysql> select SNAME,AGE from student where AGE not in (22,23);
Empty set (0.00 sec)

#取出性别为“男”，年级大于0且小于等于22岁并且学号为5的学生名字.
mysql> select * from student;
+-----+--------+------+------+
| SNO | SNAME  | AGE  | SEX  |
+-----+--------+------+------+
| 1   | 李强   |   23 | 男   |
| 2   | 刘丽   |   22 | 女   |
| 5   | 张友   |   22 | 男   |
+-----+--------+------+------+
3 rows in set (0.00 sec)

mysql> select SNAME from student where SEX="男" and (AGE > 0 and AGE <=22) and SNO = 5;
+--------+
| SNAME  |
+--------+
| 张友   |
+--------+
1 row in set (0.00 sec)

注意：多个条件直接使用“()”括起来,"and"的优先级大于"or"，条件过多时会发生歧义。

#mysql种like的用法,查找SNAME像“%强”的学生,“%”匹配任意字符.
mysql> select SNAME from student where SNAME like "%强";
+--------+
| SNAME  |
+--------+
| 李强   |
+--------+
1 row in set (0.00 sec)

#mysql种“_”匹配单一字符.
mysql> select SNAME from student where SNAME like "_强";
+--------+
| SNAME  |
+--------+
| 李强   |
+--------+
1 row in set (0.00 sec)

#将student表中的名字像“_强”的改为“换换”.
mysql> update student set SNAME = "换换" where SNAME like "_强";
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> select * from student;
+-----+--------+------+------+
| SNO | SNAME  | AGE  | SEX  |
+-----+--------+------+------+
| 1   | 换换   |   23 | 男   |
| 2   | 刘丽   |   22 | 女   |
| 5   | 张友   |   22 | 男   |
+-----+--------+------+------+
3 rows in set (0.00 sec)
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