1.字符串函數(shù)
1.1 CONCAT 函數(shù) 把傳入的參數(shù)連接成一個字符串,但是任何字符串與NULL連接都會是NULL
例子：
select concat('aaa','bbb','ccc'),concat('aaa',null);
+---------------------------+--------------------+
| concat('aaa','bbb','ccc') | concat('aaa',null) |
+---------------------------+--------------------+
| aaabbbccc | NULL |
+---------------------------+--------------------+
1.2 INSERT(str,x,y,instr)函數(shù)：將字符串 str 從第x位置開始,y個字符長的子串替換為字符串 instr食棕。
例子：把字符串"beijing2008you" 中從第12個字符串開始以后的3個字符替換成"me"混萝。
select INSERT('beijing2008you',12,3,'me');
+------------------------------------+
| INSERT('beijing2008you',12,3,'me') |
+------------------------------------+
| beijing2008me |
+------------------------------------+
1.3 LOWER(str)小寫 和 UPPER(str)大寫 函數(shù)：把字符串轉(zhuǎn)換成小寫或大寫。
select LOWER('BEIJING2008'),UPPER('beijing2008');
+----------------------+----------------------+
| LOWER('BEIJING2008') | UPPER('beijing2008') |
+----------------------+----------------------+
| beijing2008 | BEIJING2008 |
+----------------------+----------------------+
1.4 LEFT(str,x) 和 RIGHT(str,x)函數(shù)：分別返回字符串最左邊的x個字符和最右邊的x個字符工猜，如果第二個參數(shù)是 NULL，那么將不返回任何字符串男杈。
select left('beijing2008',7),left('beijing',null),right('beijing',4);
+-----------------------+----------------------+--------------------+
| left('beijing2008',7) | left('beijing',null) | right('beijing',4) |
+-----------------------+----------------------+--------------------+
| beijing | NULL | jing |
+-----------------------+----------------------+--------------------+
1.5 LPAD(str,n,pad) 和 RPAD(str,n,pad)函數(shù)：用字符串pad對str 最左邊和最右邊進(jìn)行填充挂脑，直到長度為n個字符長度迫肖。
select lpad('2008',20,'beijing'),rpad('beijing',20,'2008');
+---------------------------+---------------------------+
| lpad('2008',20,'beijing') | rpad('beijing',20,'2008') |
+---------------------------+---------------------------+
| beijingbeijingbe2008 | beijing2008200820082 |
+---------------------------+---------------------------+
1.6 LTRIM(str)和RTRIM(str)函數(shù)：去掉字符串str左側(cè)和右側(cè)空格
select ltrim(' |beijing'),rtrim('beijing| ');
+--------------------+---------------------+
| ltrim(' |beijing') | rtrim('beijing| ') |
+--------------------+---------------------+
| |beijing | beijing| |
+--------------------+---------------------+
1.7 REPEAT(str,x)函數(shù)：返回str重復(fù)x次的結(jié)果拷呆。
select repeat('mysql',3);
+-------------------+
| repeat('mysql',3) |
+-------------------+
| mysqlmysqlmysql |
+-------------------+
1.8 REPLACE(str,a,b)函數(shù)：用字符串b替換字符串str中所有出現(xiàn)的字符串a(chǎn)闲坎。
select replace('beijing2008','i','s');
+--------------------------------+
| replace('beijing2008','i','s') |
+--------------------------------+
| besjsng2008 |
+--------------------------------+
1.9 STRCMP(s1,s2)函數(shù)：比較字符串ASCII碼值的大小。
如果s1比s2小茬斧，那么返回-1箫柳，如果s1和s2相等，那么返回0啥供，如果s1大于s2，那么返回1
select strcmp('a','b'),strcmp('b','b'),strcmp('c','b');
+-----------------+-----------------+-----------------+
| strcmp('a','b') | strcmp('b','b') | strcmp('c','b') |
+-----------------+-----------------+-----------------+
| -1 | 0 | 1 |
+-----------------+-----------------+-----------------+
1.10 TRIM(str) 函數(shù)：去掉目標(biāo)字符串的開頭和結(jié)尾的空格库糠。
select trim(' $aaa$ ');
+------------------+
| trim(' $aaa$ ') |
+------------------+
| $aaa$ |
+------------------+
1.11 SUBSTRING(str,x,y)函數(shù)：返回從字符串str中的第x位置起y個字符長度的字串
select substring('beijing2008',8,4),substring('beijing2008',1,7);
+------------------------------+------------------------------+
| substring('beijing2008',8,4) | substring('beijing2008',1,7) |
+------------------------------+------------------------------+
| 2008 | beijing |
+------------------------------+------------------------------+
2.數(shù)值函數(shù)
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3.日期和時間函數(shù)
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curdate() 返回當(dāng)前日期 只包含年伙狐、月涮毫、日
select curdate();
+------------+
| curdate() |
+------------+
| 2017-09-15 |
+------------+
curtime() 反當(dāng)前時間 只包含 時分秒
select curtime();
+-----------+
| curtime() |
+-----------+
| 08:45:52 |
now() 返回當(dāng)前時間，包含年月日時分秒
select now();
+---------------------+
| now() |
+---------------------+
| 2017-09-15 08:46:24 |
+---------------------+
unix_timestamp(now()) 返回日期date的unix時間戳
select unix_timestamp(now());
+-----------------------+
| unix_timestamp(now()) |
+-----------------------+
| 1505436536 |
+-----------------------+
from_unixtime 返回時間戳的日期值
select from_unixtime(1184134516);
+---------------------------+
| from_unixtime(1184134516) |
+---------------------------+
| 2007-07-11 14:15:16 |
+---------------------------+
week(date) 返回的是所給日期中一年中的第幾周 year(date)函數(shù)返回所給的日期是哪一年
select week(now()),year(now());
+-------------+-------------+
| week(now()) | year(now()) |
+-------------+-------------+
| 37 | 2017 |
+-------------+-------------+
hour(time) 返回小時 minute(time) 返回分鐘
select hour(now()),minute(now());
+-------------+---------------+
| hour(now()) | minute(now()) |
+-------------+---------------+
| 9 | 2 |
+-------------+---------------+
4.流程函數(shù)
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以此表實驗流程函數(shù)
+--------+--------+
| userid | salary |
+--------+--------+
| 1 | 1000 |
| 2 | 2000 |
| 3 | 3000 |
| 4 | 4000 |
| 5 | 5000 |
| 1 | NULL |
+--------+--------+
if(value,t,f) 如果value是真贷屎，返回t,否則返回f
select if(salary>2000,'high','low') from salary;
+------------------------------+
| if(salary>2000,'high','low') |
+------------------------------+
| low |
| low |
| high |
| high |
| high |
| low |
+------------------------------+
ifnull(value1,value2) 如果value1不為空罢防，返回value1，否則返回value2
select ifnull(salary,0) from salary;
+------------------+
| ifnull(salary,0) |
+------------------+
| 1000 |
| 2000 |
| 3000 |
| 4000 |
| 5000 |
| 0 |
+------------------+
select case when salary<=2000 then 'low' else 'high' end from salary;
+---------------------------------------------------+
| case when salary<=2000 then 'low' else 'high' end |
+---------------------------------------------------+
| low |
| low |
| high |
| high |
| high |
| high |
+---------------------------------------------------+
select case salary when 1000 then 'low' when 2000 then 'mid' else 'high' end from salary;
+-----------------------------------------------------------------------+
| case salary when 1000 then 'low' when 2000 then 'mid' else 'high' end |
+-----------------------------------------------------------------------+
| low |
| mid |
| high |
| high |
| high |
| high |
+-----------------------------------------------------------------------+
5.其他常用函數(shù)
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唉侄，
咒吐，
，
属划，
恬叹，
，
同眯，
绽昼，
，
须蜗，
硅确，
，
明肮，
菱农，
，
柿估，
循未，
