运算符###########################################
=,!=(<>),>,>=,<,<= is null , is not null, isnull(expr) expr between min and max expr in(v1,v2,...)

流程#############################################
mysql> select ifnull(1,0); #如果第一个参数为NULL，则返回第二个参数值，否则返回原值.
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mysql>select case 1 when then 'one' when 2 when 'two' else 'more' end;
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mysql>select if(1>0,'ok','no') #如果条件为真则返回OK，否则返回NO.
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mysql>select nullif(1,2) #如果exp1 = exp2则返回NULL，否则返回exp1
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字符串###########################################
mysql>select ASCII('a'); #返回字符串最左边那个字符的ASCII码值
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mysql>select oct(12); #返回字符串表示的八进制形式.
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mysql>select hex(255); #返回字符串的十六进制形式.
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mysql>select char(3333342); #返回ASSCII码值对应的字符串
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mysql>select char_length('text') #返回字符串的个数
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mysql>select length(text) #返回字符串的字节数
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mysql>select concat('str1','str2','str3','str4','strn',) #将参数连接成字符串返回
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mysql>select locate('字串','母串') #返回字串在母串中第一次出现的位置，如果不存在则返回0
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mysql>select lpad('str',11,'.');
mysql>select rpad('str',11,'.'); #用字符串对str进行左边或右边填充直到他的长度达到11.
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mysql> select left('strstrstr',5); #返回字符串最左边的N个字符. mysql>select right('strstrstr',5); #返回字符串最右边的N个字符.
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mysql> select substring('xxxxxx',3,3); #从字符串的第三个字个位置起截取三个字符.
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mysql> select ltrim(str);
mysql>select rtrim();
mysql> select trim(' str ');
mysql> select trim(both 'x' from 'xxx..xxx'); #去空格
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mysql> select space(6); #返回门个空格
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mysql> select repeat('str',3); #返回重复三次的字符串
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mysql>select replace('www.tom.com','tom','sina'); #字符串替换
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mysql>select reverse('abc'); #字符串颠倒
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mysql>select lower('UPPER');
mysql>select upper('lower'); #转换大小写
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mysql> update table set filed = load_file('/tmp/file'); where id=1; #读入文件
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mysql> select abs(-1); #返回绝对值
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mysql> select 4 div 2; #除
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mysql> select 5 mod 2; #余
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mysql> select floor(x) #返回不大于X的最大整数值
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mysql> select ceiling(x) #返回大于X的最小整数
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mysql> select round(2.23) #四舍五入
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时间 mysql>select now(); select now() + 0; #返回时间
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mysql>select curdate(); select curdate() + 0; #返回年月日
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mysql>select curtime(); select curtime() + 0 #返回小时分秒
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mysql>select database() #返回当前数据库的名子
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mysql>select user();
select system_user();
select session_user(); #返回当前MYSQL用户名子
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mysql>select password('abc'); #口令加密
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删除所有表记录 #truncate table 表名;
#速度快于delete from table