【Mysql 学习】流程函数

--流程函数。if ，ifnull ，case 语句！
mysql create table sal (id int,sal decimal (9,2));
Query OK, 0 rows affected (0.06 sec)
mysql insert into sal values (1,1000),(2,2000),(3,3000),(4,4000),(5,null);
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0
--if(EXP,T,F) EXP 为真则返回T,EXP为假，则返回F
mysql select * from sal;
+------+---------+
| id   | sal     |
+------+---------+
|    1 | 1000.00 |
|    2 | 2000.00 |
|    3 | 3000.00 |
|    4 | 4000.00 |
|    5 |    NULL |
+------+---------+
5 rows in set (0.00 sec)

mysql select id, if(sal 3000,high,low) from sal;
+------+-----------------------------+
| id   | if(sal 3000,high,low) |
+------+-----------------------------+
|    1 | low                         |
|    2 | low                         |
|    3 | low                         |
|    4 | high                        |
|    5 | low                         |--这里null 3000 为假！
+------+-----------------------------+
5 rows in set (0.01 sec)
--IFNULL(VAL,N)如果val 为null 则返回N
mysql select id, ifnull(sal,0) from sal;
+------+---------------+
| id   | ifnull(sal,0) |
+------+---------------+
|    1 |       1000.00 |
|    2 |       2000.00 |
|    3 |       3000.00 |
|    4 |       4000.00 |
|    5 |          0.00 |
+------+---------------+
5 rows in set (0.01 sec)
--case 语句和oracle的一样了！
mysql select case when sal =3000 then low else high end from sal;
+------------------------------------------------+
| case when sal =3000 then low else high end |
+------------------------------------------------+
| low                                            |
| low                                            |
| low                                            |
| high                                           |
| high                                           |
+------------------------------------------------+
5 rows in set (0.00 sec)
mysql select id, case sal when 2000 then low when 3000 then mid else high end from sal;
+------+--------------------------------------------------------------------+
| id   | case sal when 2000 then low when 3000 then mid else high end |
+------+--------------------------------------------------------------------+
|    1 | high                                                               |
|    2 | low                                                                |
|    3 | mid                                                                |
|    4 | high                                                               |
|    5 | high                                                               |
+------+--------------------------------------------------------------------+
5 rows in set (0.01 sec)

这里只是简单介绍，跟多的需要在实践中学习！抛砖引玉了，呵呵

深入理解MySQL主从复制原理以及集群部署流程 主从复制是指将主数据库的 DDL 和 DML 操作通过二进制日志传到从库服务器中，然后在从库上对这些日志重新执行（也叫重做），从而使得从库和主库的数据保持同步。 MySQL支持一台主库同时向多台从库进行复制， 从库同时也可以作为其他从服务器的主库，实现链状复制。