mysql 函数（操作符讲解）
mysql 函数
操作符有优先级
如果想改变优先级执行顺序，可以使用括号
select 1+2*3
select （1+2）*3
符号代表
对比操作符的执行结果为true，false，null三种
- between a and b 代表检查值是否在a和b之间
- coalesce() 代表返回第一个非null的值
- =代表相等操作符
- >代表大于操作符
- >=代表大于等于操作符
- greatest（）代表返回最大的值
- in（）代表检查值是否在一系列的值之中
- interval（）代表返回比第一个参数小的参数的位置
- is/is not代表检查值是否与布尔值相同/不同
- Is not null代表检查值是否是非NULL
- Is null代表检查值是否是NULL
- Isnull()代表检查参数是NULL
- Least()代表返回最小的参数
- <代表小于操作符
- <=代表小于等于操作符
- Like代表字符（通配符）匹配
- Not between A and B代表检查值是否不在A和B的范围之内
- !=/<>代表不等于操作符
- Not in()代表检查值是否不在一系列值的当中
- Not like代表检查值是否不匹配
- Strcmp()对比两个字符串
例：
- select * from students where sid=1； ##找出sid等于1的值
- select * from students where sid<>1; ##找出sid不等于1的值
- select * from students where sid!=1; ##也是找出sid不等于1的值
- mysql> select * from students ; ##sname2是有值得
+--------+-------+---------+--------+
| sid | sname | dept_id | sname2 |
+--------+-------+---------+--------+
| 1 | bc | 1 | NULL |
| 2 | aaa | 2 | NULL |
| 3 | aa | 1 | NULL |
| 11 | bbb | 1 | NULL |
| 100001 | c | 1 | NULL |
| 980001 | a | 1 | NULL |
| 980002 | a | 1 | NULL |
| 990001 | c | 1 | NULL |
+--------+-------+---------+--------+
8 rows in set (0.00 sec)5、mysql> select * from students where sname2=null; ##单独查找出sname2又是空的（不能写=null）
Empty set (0.01 sec)
6、select * from students where sname2 is null; ##可以写成 is null
7、select * from students where sname2 is not null; ##或者 is not null
8、mysql> SELECT 1 = 0; ##返回0就代表false
-> 0
9、mysql> SELECT '0' = 0; ##字符‘0’会被转换成0再和0作对比，返回true
-> 1
10、mysql> SELECT '0.0' = 0;
-> 1
11、mysql> SELECT '0.01' = 0;
-> 0
12、mysql> SELECT '.01' = 0.01;
-> 1
<>/!=号（不相等）对比操作符
13、mysql> SELECT '.01' <> '0.01';
-> 1
14、mysql> SELECT .01 <> '0.01';
-> 0
15、mysql> SELECT 'zapp' <> 'zappp';
-> 1
is操作符
• mysql> SELECT 1 IS TRUE, 0 IS FALSE, NULL IS UNKNOWN;
• -> 1, 1, 1
Is not操作符
• mysql> SELECT 1 IS NOT UNKNOWN, 0 IS NOT UNKNOWN, NULL IS
NOT UNKNOWN;
• -> 1, 1, 0
• Is null对比操作符
• mysql> SELECT 1 IS NULL, 0 IS NULL, NULL IS NULL;
• -> 0, 0, 1
• Is not null对比操作符
• mysql> SELECT 1 IS NOT NULL, 0 IS NOT NU
• -> 1, 1, 0
• Expr between min and max对比操作符
• 相当于min <= expr AND expr <= max
• mysql> SELECT 2 BETWEEN 1 AND 3, 2 BETWEE
• -> 1, 0
• mysql> SELECT 1 BETWEEN 2 AND 3;
• -> 0
• mysql> SELECT 'b' BETWEEN 'a' AND 'c';
• -> 1
• mysql> SELECT 2 BETWEEN 2 AND '3';
• -> 1
• mysql> SELECT 2 BETWEEN 2 AND 'x-3';
• -> 0
• expr NOT BETWEEN min AN
• 相当于NOT (expr BETWEE
• COALESCE(value,...)对比操
• 返回第一个非NULL的值，
• mysql> SELECT COALESCE(
• -> 2
• mysql> SELECT COALESCE(
• -> NULL
• GREATEST(value1,value2,...
• 返回其中最大的值
• mysql> SELECT GREATEST(
• -> 2
• mysql> SELECT GREATEST(
• -> 767.0
• mysql> SELECT GREATEST(
• -> 'C'
expr IN (value,...)对比操作符
• 当expr值能在values中找到，则返回1，否则返回0
• mysql> SELECT 2 IN (0,3,5,7);
• -> 0
• mysql> SELECT 'wefwf' IN ('wee','wefwf','weg');
• -> 1
• mysql> SELECT (3,4) IN ((1,2), (3,4));
• -> 1
• mysql> SELECT (3,4) IN ((1,2), (3,5));
• -> 0
• expr NOT IN (value,...)对比操作符
• Isnull(expr)操作符
• 如果expr是null，则
• mysql> SELECT ISNUL
• -> 0
• mysql> SELECT ISNUL
• -> 1
• LEAST(value1,value2,
• 返回最小值，如果其
• mysql> SELECT LEAST
• -> 0
• mysql> SELECT LEAST
• -> 3.0
• mysql> SELECT LEAST
• -> 'A'
逻辑操作符返回1 (TRUE), 0 (FALSE), 或者NULL
• NOT, !逻辑操作符代表非操作
• mysql> SELECT NOT 10;
• -> 0
• mysql> SELECT NOT 0;
• -> 1
• mysql> SELECT NOT NULL;
• -> NULL
• mysql> SELECT ! (1+1);
• -> 0
• mysql> SELECT ! 1+1;
• -> 1
• And,&&逻辑操作符
• mysql> SELECT 1 AND 1;
• -> 1
• mysql> SELECT 1 AND 0;
• -> 0
• mysql> SELECT 1 AND NULL;
• -> NULL
• mysql> SELECT 0 AND NULL;
• -> 0
• mysql> SELECT NULL AND 0;
• -> 0
例：select * from students where sid=2 and sname='bc'; ##必须满足sid=2和sname=‘bc’两个条件，不然后返回空值
select * from students where sid=2 or sname='bc'; ##满足其中一个条件，就会返回满足条件的值（两个满足了条件的值都会返回）
分配操作符
分配操作符做赋值操作
:=是赋值操作 =在有些情况下也是赋值操作
例：
mysql> select @a; ##定义变量
+------+
| @a |
+------+
| NULL |
+------+
1 row in set (0.00 sec)mysql> set @a:=1; ##赋值
Query OK, 0 rows affected (0.01 sec)
mysql> select @a; ##查询变量
+------+
| @a |
+------+
| 1 |
+------+
1 row in set (0.01 sec)set @a:='a'; ##赋值
定义变量并且赋值
select @a:=count(*) from students; ##把select count(*) from students;得出的值赋予@a
update students set sname='abc' where sid=1; ##在update中赋值是=号
##在一般的操作中是:=做赋值
• mysql> SELECT @var1, @var2; ##@后跟上字符或者字符串可以定义变量
• -> NULL, NULL
• mysql> SELECT @var1 := 1, @var2;
• -> 1, NULL
• mysql> SELECT @var1, @var2;
• -> 1, NULL
• mysql> SELECT @var1, @var2 := @var1;
• -> 1, 1
• mysql> SELECT @var1, @var2;
• -> 1, 1
• mysql> SELECT @var1:=COUNT(*) FROM t1;
• -> 4
• mysql> SELECT @var1;
• -> 4
01
流程控制函数
流程控制函数包含以下四种函数（逻辑判断）
case和if作用相似写法不一
CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result ...] [ELSE result] END
##如果value等于 [compare_value]就返回result，如果不等于这个值是等于下一个[compare_value]时候就返回result
课件
CASE value WHEN [compare_value] THEN result [WHEN [compare_value] THEN result ...] [ELSE result] END
##当value等于compare_value时，则返回result，否则返回else里的result，如果没有else子句则返回null
例：
mysql> select case 1 when 1 then 'aa' when 2 then 'bb' else 'cc' end;
+--------------------------------------------------------+
| case 1 when 1 then 'aa' when 2 then 'bb' else 'cc' end |
+--------------------------------------------------------+
| aa |
+--------------------------------------------------------+
1 row in set (0.00 sec)
##如果 case 1等于1就返回aa，如果等于2就返回bb，如果它不等于1也不等于2就返回cc
mysql> select case dept_id when 1 then 'english' when 2 then 'accounting' else 'unknown' end from students;
+---------------------------------------------------------------------------------+
| case dept_id when 1 then 'english' when 2 then 'accounting' else 'unknown' end |
+---------------------------------------------------------------------------------+
| english |
| english |
| english |
| english |
| english |
| english |
| english |
| accounting |
+---------------------------------------------------------------------------------+
8 rows in set (0.01 sec)
##查询students表中dept_id等于1就返回English如果等于2就返回accounting，都没有就返回unknown
mysql> select dept_id,case dept_id when 1 then 'english' when 2 then 'accounting' else 'unknown' end from students;
+---------+---------------------------------------------------------------------------------+
| dept_id | case dept_id when 1 then 'english' when 2 then 'accounting' else 'unknown' end |
+---------+---------------------------------------------------------------------------------+
| 1 | english |
| 1 | english |
| 1 | english |
| 1 | english |
| 1 | english |
| 1 | english |
| 1 | english |
| 2 | accounting |
+---------+---------------------------------------------------------------------------------+
8 rows in set (0.01 sec)
case的另一种写法
select sname,case when dept_id>=1 and dept_id<2 then 'english' when dept_id>=2 then 'accounting' else 'unknown' end from students;
+-------+--------------------------------------------------------------------------------------------------------+
| sname | case when dept_id>=1 and dept_id<2 then 'english' when dept_id>=2 then 'accounting' else 'unknown' end |
+-------+--------------------------------------------------------------------------------------------------------+
| abc | english |
| aaa | accounting |
| aa | english |
| bbb | english |
| c | english |
| a | english |
| a | english |
| c | english |
+-------+--------------------------------------------------------------------------------------------------------+
8 rows in set (0.00 sec)
mysql> select sname,case when dept_id>=1 and dept_id<=2 then 'english' when dept_id>=2 then 'accounting' else 'unknown' end,dept_id from students;
##当1行判断满足多个条件就返回第一个条件的值，不会往下匹配了
+-------+---------------------------------------------------------------------------------------------------------+---------+
| sname | case when dept_id>=1 and dept_id<=2 then 'english' when dept_id>=2 then 'accounting' else 'unknown' end | dept_id |
+-------+---------------------------------------------------------------------------------------------------------+---------+
| abc | english | 1 |
| aaa | english | 2 |
| aa | english | 1 |
| bbb | english | 1 |
| c | english | 1 |
| a | english | 1 |
| a | english | 1 |
| c | english | 1 |
+-------+---------------------------------------------------------------------------------------------------------+---------+
8 rows in set (0.01 sec)
流程控制if
if有三个参数
例：
mysql> select sname,if(dept_id>=2,'english','accounting'),dept_id from students; ##dept_id大于等于2显示English，不是就显示accounting
+-------+---------------------------------------+---------+
| sname | if(dept_id>=2,'english','accounting') | dept_id |
+-------+---------------------------------------+---------+
| bc | accounting | 1 |
| aaa | english | 2 |
| aa | accounting | 1 |
| c | accounting | 1 |
| a | accounting | 1 |
| a | accounting | 1 |
| c | accounting | 1 |
+-------+---------------------------------------+---------+
7 rows in set (0.00 sec)
mysql> select sname,if(dept_id>=1,'english','accounting'),dept_id from students;
+-------+---------------------------------------+---------+
| sname | if(dept_id>=1,'english','accounting') | dept_id |
+-------+---------------------------------------+---------+
| bc | english | 1 |
| aaa | english | 2 |
| aa | english | 1 |
| c | english | 1 |
| a | english | 1 |
| a | english | 1 |
| c | english | 1 |
+-------+---------------------------------------+---------+
7 rows in set (0.00 sec)
流程控制语句nullif（不经常用到）
例：
mysql> select nullif(1,1); ##1等于1就返回null
+-------------+
| nullif(1,1) |
+-------------+
| NULL |
+-------------+
1 row in set (0.00 sec)
mysql> select nullif(1,2); ##1不等于2就返回第一个表达式1
+-------------+
| nullif(1,2) |
+-------------+
| 1 |
+-------------+
1 row in set (0.00 sec)
字符串函数
ASCII（转换）
返回str字符串中最左边字符的ASCII码值，如果是空串则返回0，如果str是null则返回null
mysql> select ascii('2');
+------------+
| ascii('2') |
+------------+
| 50 |
+------------+
1 row in set (0.02 sec)
mysql> select ascii('b'); ##字符转换成数字
+------------+
| ascii('b') |
+------------+
| 98 |
+------------+
1 row in set (0.00 sec)
将字符转换成ascii码
例：
mysql> select char('97'); ##数字转换成字符
+------------+
| char('97') |
+------------+
| a |
+------------+
1 row in set (0.00 sec)
经常用到
char_length返回字符长度而不是字节长度
例：
mysql> select char_length('abc');
+--------------------+
| char_length('abc') |
+--------------------+
| 3 |
+--------------------+
1 row in set (0.00 sec)
mysql> select * from students where char_length(sname)=3; ##找出students表中sname字符等于3的值
+-----+-------+--------+---------+--------+
| sid | sname | gender | dept_id | sname1 |
+-----+-------+--------+---------+--------+
| 2 | aaa | 2 | 2 | NULL |
+-----+-------+--------+---------+--------+
1 row in set (0.00 sec)
concat
例：
mysql> select concat('my','s','ql'); ##把字符串拼到一起
+-----------------------+
| concat('my','s','ql') |
+-----------------------+
| mysql |
+-----------------------+
1 row in set (0.00 sec)
mysql> select concat('my',null,'ql'); ##字符串中有一个值是null，就返回null
+------------------------+
| concat('my',null,'ql') |
+------------------------+
| NULL |
+------------------------+
1 row in set (0.00 sec)
mysql> select concat(sname1,sname) from students; ##字段sname1和sname的拼接
+----------------------+
| concat(sname1,sname) |
+----------------------+
| wubc |
| wangaaa |
| NULL |
| c |
| NULLa |
| NULLa |
| NULLc |
+----------------------+
7 rows in set (0.00 sec)
mysql> select concat(sname1,' ',sname) from students; ##字段sname1和sname的拼接，中间加个空格
mysql> select concat(ifnull(sname1,''),' ',sname) from students;
+-------------------------------------+
| concat(ifnull(sname1,''),' ',sname) |
+-------------------------------------+
| wu bc |
| wang aaa |
| aa |
| c |
| NULL a |
| NULL a |
| NULL c |
+-------------------------------------+
7 rows in set (0.00 sec)
concat_ws 规定了用什么分隔符连接各个字符串，遇到null值会把null忽略
mysql> select concat_ws(',',sname1,sname) from students; ##用逗号做分隔符
+-----------------------------+
| concat_ws(',',sname1,sname) |
+-----------------------------+
| wu,bc |
| wang,aaa |
| aa |
| ,c |
| NULL,a |
| NULL,a |
| NULL,c |
+-----------------------------+
7 rows in set (0.00 sec)
mysql> select concat(ifnull(sname1,''),',',sname) from students; ##ifnull也用逗号做分割
+-------------------------------------+
| concat(ifnull(sname1,''),',',sname) |
+-------------------------------------+
| wu,bc |
| wang,aaa |
| ,aa |
| ,c |
| NULL,a |
| NULL,a |
| NULL,c |
+-------------------------------------+
7 rows in set (0.00 sec)