MySQL管理与优化(4)_MySQL

常用函数

字符串函数：

• 常用的字符串函数

    

• CONCAT(s1, s2, ...):依次连接字符串。  

mysql> SELECT CONCAT('aa', 'bb'), CONCAT('abc', 'de', 'fgh');+--------------------+----------------------------+| CONCAT('aa', 'bb') | CONCAT('abc', 'de', 'fgh') |+--------------------+----------------------------+| aabb               | abcdefgh                   |+--------------------+----------------------------+

• INSERT(str, x, y, instr):将str从x位置开始的y个字符串替换为instr，索引从1开始。

mysql> SELECT INSERT('ilike2014', 2, 4, 'enjoy');+------------------------------------+| INSERT('ilike2014', 2, 4, 'enjoy') |+------------------------------------+| ienjoy2014                         |+------------------------------------+

• LPAD(str, n, pad):str长度不足n，则左填充pad。

mysql> SELECT LPAD('XXX', 12, '01');+-----------------------+| LPAD('XXX', 12, '01') |+-----------------------+| 010101010XXX          |+-----------------------+

• SUBSTRING(str, x, y): 求子串str[x..y]，索引从1开始。

mysql> SELECT SUBSTRING('abcdefg', 1, 3);+----------------------------+| SUBSTRING('abcdefg', 1, 3) |+----------------------------+| abc                        |+----------------------------+

数值函数：

• MySQL常用的数值函数：

    

    范例：

mysql> SELECT ABS(0.8), ABS(-0.8), CEIL(-0.8), CEIL(0.8), FLOOR(-0.8), FLOOR(0.8), MOD(15,10), MOD(1,11), MOD(NULL,10);+----------+-----------+------------+-----------+-------------+------------+------------+-----------+--------------+| ABS(0.8) | ABS(-0.8) | CEIL(-0.8) | CEIL(0.8) | FLOOR(-0.8) | FLOOR(0.8) | MOD(15,10) | MOD(1,11) | MOD(NULL,10) |+----------+-----------+------------+-----------+-------------+------------+------------+-----------+--------------+|      0.8 |       0.8 |          0 |         1 |          -1 |          0 |          5 |         1 |         NULL |+----------+-----------+------------+-----------+-------------+------------+------------+-----------+--------------+mysql> SELECT ROUND(1.235, 2), TRUNCATE(1.235, 2);+-----------------+--------------------+| ROUND(1.235, 2) | TRUNCATE(1.235, 2) |+-----------------+--------------------+|            1.24 |               1.23 |+-----------------+--------------------+

日期和时间函数：

• 常用的日期时间函数：

    

    范例：

-- 列举当前时间mysql> SELECT CURDATE(), CURTIME(), NOW();+------------+-----------+---------------------+| CURDATE()  | CURTIME() | NOW()               |+------------+-----------+---------------------+| 2014-06-12 | 11:54:22  | 2014-06-12 11:54:22 |+------------+-----------+---------------------+-- 返回Unix时间戳mysql> SELECT UNIX_TIMESTAMP(now());+-----------------------+| UNIX_TIMESTAMP(now()) |+-----------------------+|            1402545326 |+-----------------------+-- 返回Unix时间戳对应的日期mysql> SELECT UNIX_TIMESTAMP(now());+-----------------------+| UNIX_TIMESTAMP(now()) |+-----------------------+|            1402545326 |+-----------------------+-- 现在是哪周，哪年mysql> SELECT WEEK(now()), YEAR(now());+-------------+-------------+| WEEK(now()) | YEAR(now()) |+-------------+-------------+|          23 |        2014 |+-------------+-------------+

• MySQL中的日期时间格式化：

    

    如：

mysql> SELECT DATE_FORMAT(now(), '%Y-%m-%d %H:%m:%s');+-----------------------------------------+| DATE_FORMAT(now(), '%Y-%m-%d %H:%m:%s') |+-----------------------------------------+| 2014-06-12 12:06:13                     |+-----------------------------------------+

• 有关时间计算

       MySQL中的日期时间间隔类型有：

       

       范例：

mysql> SELECT now() current, DATE_ADD(now(), INTERVAL 31 DAY) after31days, DATE_ADD(now(), INTERVAL -7 DAY) before7days, DATE_ADD(now(), INTERVAL '1_2' YEAR_MONTH) after_1year_2month;+---------------------+---------------------+---------------------+---------------------+| current             | after31days         | before7days         | after_1year_2month  |+---------------------+---------------------+---------------------+---------------------+| 2014-06-12 22:48:49 | 2014-07-13 22:48:49 | 2014-06-05 22:48:49 | 2015-08-12 22:48:49 |+---------------------+---------------------+---------------------+---------------------+

流程函数：

• MySQL中的流程函数：

    

    范例：

-- 初始化数据mysql> CREATE TABLE salary (userid int, salary decimal(9, 2));Query OK, 0 rows affected (0.06 sec)mysql> INSERT INTO salary VALUES(1, 1000), (2, 2000), (3, 3000), (4, 4000), (5, 5000), (1, NULL);Query OK, 6 rows affected (0.02 sec)Records: 6  Duplicates: 0  Warnings: 0mysql> SELECT * FROM salary;+--------+---------+| userid | salary  |+--------+---------+|      1 | 1000.00 ||      2 | 2000.00 ||      3 | 3000.00 ||      4 | 4000.00 ||      5 | 5000.00 ||      1 |    NULL |+--------+---------+-- 根据薪水高低判断薪水mysql> SELECT IF(salary>2000, 'high', 'low') FROM salary;+--------------------------------+| IF(salary>2000, 'high', 'low') |+--------------------------------+| low                            || low                            || high                           || high                           || high                           || low                            |+--------------------------------+-- IFNULL使用mysql> SELECT IFNULL(salary, 0) FROM salary;+-------------------+| IFNULL(salary, 0) |+-------------------+|           1000.00 ||           2000.00 ||           3000.00 ||           4000.00 ||           5000.00 ||              0.00 |+-------------------+-- CASE WHEN使用mysql> 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                                                                  |+-----------------------------------------------------------------------+

其他常用函数：

• MySQL中其他常用函数：

    

    范例：

mysql> SELECT DATABASE(), VERSION(), USER();+------------+-----------+----------------+| DATABASE() | VERSION() | USER()         |+------------+-----------+----------------+| test       | 5.6.14    | root@localhost |+------------+-----------+----------------+mysql> SELECT DATABASE(), VERSION(), USER();+------------+-----------+----------------+| DATABASE() | VERSION() | USER()         |+------------+-----------+----------------+| test       | 5.6.14    | root@localhost |+------------+-----------+----------------+mysql> SELECT INET_ATON('192.168.141.129');+------------------------------+| INET_ATON('192.168.141.129') |+------------------------------+|                   3232271745 |+------------------------------+-- 41位加密密码mysql> SELECT PASSWORD('111111');+-------------------------------------------+| PASSWORD('111111')                        |+-------------------------------------------+| *FD571203974BA9AFE270FE62151AE967ECA5E0AA |+-------------------------------------------+-- MD5值计算mysql> SELECT MD5('111111');+----------------------------------+| MD5('111111')                    |+----------------------------------+| 96e79218965eb72c92a549dd5a330112 |+----------------------------------+

MySQL更多函数可参考：

http://dev.mysql.com/doc/refman/5.7/en/functions.html

不吝指正。