mysql函数和操作符

mysql> select mod(29,9);    //取模函数
+-----------+
| mod(29,9) |
+-----------+
|         2 |
+-----------+
1 row in set (0.00 sec)

mysql> select mod(29,2);
+-----------+
| mod(29,2) |
+-----------+
|         1 |
+-----------+
1 row in set (0.00 sec)

mysql> select 2 between 1 and 10;      //between xx  and  xx
+--------------------+
| 2 between 1 and 10 |
+--------------------+
|                  1 |
+--------------------+
1 row in set (0.06 sec)

mysql> select 20 between 1 and 10;
+---------------------+
| 20 between 1 and 10 |
+---------------------+
|                   0 |
+---------------------+
1 row in set (0.00 sec)

mysql> select 20 not between 1 and 10;    //not  between xx  and  xx
+-------------------------+
| 20 not between 1 and 10 |
+-------------------------+
|                       1 |
+-------------------------+
1 row in set (0.00 sec)

mysql> select 20 not between 1 and 50;
+-------------------------+
| 20 not between 1 and 50 |
+-------------------------+
|                       0 |
+-------------------------+
1 row in set (0.00 sec)

mysql> select greatest(1,2,56,7,5); //greatest函数，在一串数字中取最大值
+----------------------+
| greatest(1,2,56,7,5) |
+----------------------+
|                   56 |
+----------------------+
1 row in set (0.08 sec)

mysql> select greatest('a','b','c','d','e');//字母取最大值
+-------------------------------+
| greatest('a','b','c','d','e') |
+-------------------------------+
| e                             |
+-------------------------------+
1 row in set (0.35 sec)

ISNULL(expr)     //是空值
如expr 为NULL，那么ISNULL() 的返回值为 1，否则返回值为 0
mysql> select isnull(5);    //是空值，为真则返回1，否则返回0
+-----------+
| isnull(5) |
+-----------+
|         0 |
+-----------+
1 row in set (0.00 sec)

mysql> select isnull(null);;
+--------------+
| isnull(null) |
+--------------+
|            1 |
+--------------+
1 row in set (0.00 sec)

LEAST(value1,value2,...)
在有两个或多个参数的情况下， 返回值为最小 (最小值) 参数
mysql> select least(2,0,1,-9,5,4);
+---------------------+
| least(2,0,1,-9,5,4) |
+---------------------+
|                  -9 |
+---------------------+
1 row in set (0.02 sec)

mysql> select least('a','b','c','d','e');
+----------------------------+
| least('a','b','c','d','e') |
+----------------------------+
| a                          |
+----------------------------+
1 row in set (0.00 sec)

mysql> select (case 1 when 1 then 'one'   //case语法
    -> when 2 then 'two'
    -> else 'more'
    -> end) as cid;
+-----+
| cid |
+-----+
| one |
+-----+
1 row in set (0.00 sec)
//案例解析
select 'AAA',                                                                                                                            //标记1
       (case  cid   when '' then 'xxxx'   when '' then 'xxxx'   when '' then 'xxxx'  else cid  end )  as 渠道名称,    //标记2
       intdate as 注册日期,
from 表名  where intdate>= ''  and intdate<= ''
//解析
1）标记2为一个语句
2）as将语句重命名为渠道名称
3）case语法结构：(case  cid   when '' then 'xxxx'   when '' then 'xxxx'  else cid  end ) ，从cid中匹配到编号1的时候，返回的结果将1赋值为xxxx
4）else cid，当cid不为1,2时，将直接返回cid本身

//if函数
//IF(expr1,expr2,expr3)
如果 expr1 是TRUE，则 IF()的返回值为expr2; 否则返回值则为 expr3。
IF() 的返回值为数字值或字符串值，具体情况视其所在语境而定
mysql> select if(1>5,'yes','no');    //和excel中的if函数用法一致
+--------------------+
| if(1>5,'yes','no') |
+--------------------+
| no                 |
+--------------------+
1 row in set (0.00 sec)

mysql> select if(1<5,'yes','no');
+--------------------+
| if(1<5,'yes','no') |
+--------------------+
| yes                |
+--------------------+
1 row in set (0.00 sec)

//CONCAT(str1,str2,...)
mysql> select concat('my','sql');   //mysql
+--------------------+
| concat('my','sql') |
+--------------------+
| mysql              |
+--------------------+
1 row in set (0.38 sec)

mysql> select concat('my','null','sql');  //mynullsql
+---------------------------+
| concat('my','null','sql') |
+---------------------------+
| mynullsql                 |
+---------------------------+
1 row in set (0.00 sec)

mysql> select concat('my',null,'sql');  //NULL
+-------------------------+
| concat('my',null,'sql') |
+-------------------------+
| NULL                    |
+-------------------------+
1 row in set (0.00 sec)

mysql> select concat(14.3);   //14.3
+--------------+
| concat(14.3) |
+--------------+
| 14.3         |
+--------------+
1 row in set (0.00 sec)

mysql> select concat(14.3,25);   //14.325
+-----------------+
| concat(14.3,25) |
+-----------------+
| 14.325          |
+-----------------+
1 row in set (0.00 sec)

//INSTR(str,substr)
返回字符串 str 中子字符串的第一个出现位置。这和LOCATE()的双参数形式相同，除非参数的顺序被颠倒
mysql> select instr('foobarbar','bar');
+--------------------------+
| instr('foobarbar','bar') |
+--------------------------+
|                        4 |
+--------------------------+
1 row in set (0.35 sec)

mysql> select lower('MySQL');   //lower和lcase转化为小写
+----------------+
| lower('MySQL') |
+----------------+
| mysql          |
+----------------+
1 row in set (0.00 sec)

mysql> select lcase('MySQL');
+----------------+
| lcase('MySQL') |
+----------------+
| mysql          |
+----------------+
1 row in set (0.00 sec)

mysql> select left('foobar',4);   //从左向右取数据，取4个数据
+------------------+
| left('foobar',4) |
+------------------+
| foob             |
+------------------+
1 row in set (0.00 sec)

mysql> select right('foobar',4);  //从右向左取数据，取4个数据
+-------------------+
| right('foobar',4) |
+-------------------+
| obar              |
+-------------------+
1 row in set (0.36 sec)

mysql> select length('mysql');  //length求字符串的长度
+-----------------+
| length('mysql') |
+-----------------+
|               5 |
+-----------------+
1 row in set (0.00 sec)

//返回字符串 str ，其引导空格字符被删除
mysql> select ltrim('  bar') as str;   //ltrim删除左边的空格引导字符
+------+
| str  |
+------+
| bar  |
+------+
1 row in set (0.00 sec)

mysql> select rtrim('  bar  ') as str;    //rtrim删除右边的空格引导字符
+-------+
| str   |
+-------+
|   bar |
+-------+
1 row in set (0.05 sec)

mysql> select trim('  bar  ') as str;  //trim删除2边的空格引导符
+------+
| str  |
+------+
| bar  |
+------+
1 row in set (0.00 sec)

//SUBSTRING
substring(str, pos); substring(str, pos, len)
从字符串的第pos个字符位置开始取，取len个数据，直到结束。
mysql> select substring('example',4,2);
+--------------------------+
| substring('example',4,2) |
+--------------------------+
| mp                       |
+--------------------------+
1 row in set (0.00 sec)