MySQL添加可重复执行列

一、information_schema数据库表说明:

SCHEMATA表：提供了当前mysql实例中所有数据库的信息。是show databases的结果取之此表。

TABLES表：提供了关于数据库中的表的信息（包括视图）。详细表述了某个表属于哪个schema，表类型，表引擎，创建时间等信息。是show tables from schemaname的结果取之此表。

COLUMNS表：提供了表中的列信息。详细表述了某张表的所有列以及每个列的信息。是show columns from schemaname.tablename的结果取之此表。

STATISTICS表：提供了关于表索引的信息。是show index from schemaname.tablename的结果取之此表。

USER_PRIVILEGES（用户权限）表：给出了关于全程权限的信息。该信息源自mysql.user授权表。是非标准表。

SCHEMA_PRIVILEGES（方案权限）表：给出了关于方案（数据库）权限的信息。该信息来自mysql.db授权表。是非标准表。

TABLE_PRIVILEGES（表权限）表：给出了关于表权限的信息。该信息源自mysql.tables_priv授权表。是非标准表。

COLUMN_PRIVILEGES（列权限）表：给出了关于列权限的信息。该信息源自mysql.columns_priv授权表。是非标准表。

CHARACTER_SETS（字符集）表：提供了mysql实例可用字符集的信息。是SHOW CHARACTER SET结果集取之此表。

COLLATIONS表：提供了关于各字符集的对照信息。

COLLATION_CHARACTER_SET_APPLICABILITY表：指明了可用于校对的字符集。这些列等效于SHOW COLLATION的前两个显示字段。

TABLE_CONSTRAINTS表：描述了存在约束的表。以及表的约束类型。

KEY_COLUMN_USAGE表：描述了具有约束的键列。

ROUTINES表：提供了关于存储子程序（存储程序和函数）的信息。此时，ROUTINES表不包含自定义函数（UDF）。名为“mysql.proc name”的列指明了对应于INFORMATION_SCHEMA.ROUTINES表的mysql.proc表列。

VIEWS表：给出了关于数据库中的视图的信息。需要有show views权限，否则无法查看视图信息。

TRIGGERS表：提供了关于触发程序的信息。必须有super权限才能查看该表

二、存储过程实现

思路：

1.先判断表是否存在；

2.判断列名是否存在该表中；

3.判断null、default是否存在属性中；

4.判断是否需要插入到某个列名之后；

5.拼装sql语句；

6.执行添加字段的sql语句；

 drop procedure if exists `AddColumnIfNotExists`;
 delimiter $$
 create procedure AddColumnIfNotExists(in tableName varchar(50),in columnName varchar(50),in discription varchar(256), in position varchar(50))
 begin
     declare tableCount int default 0;  #用于判断表是否存在
     declare columnCount int default 0; #用于判断字段是否存在
     declare stmt varchar(2000); #最终拼接出来的sql语句
     declare seat int default 0; #用于判断null/default的位置

     SELECT count(*) into @tableCount FROM information_schema.TABLES WHERE TABLE_SCHEMA = (SELECT DATABASE()) AND TABLE_NAME = tableName;
     SELECT count(*) into @columnCount FROM information_schema.COLUMNS WHERE TABLE_SCHEMA = (SELECT DATABASE()) AND TABLE_NAME = tableName AND COLUMN_NAME = columnName;

     if @tableCount=1 && @columnCount=0 then
         set @seat = locate('null', discription);
         if  @seat > 0 then #判断是否存在null
             set @stmt = CONCAT(' ALTER TABLE `',tableName,'` ADD COLUMN `',columnName,'` ',discription);
         else
             set @seat = locate('default', discription);
             if @seat > 0 then #判断是否存在default
                 set @stmt = CONCAT(' ALTER TABLE `',tableName,'` ADD COLUMN `',columnName,'` ',discription, ' NOT NULL');
             else
                 set @stmt = CONCAT(' ALTER TABLE `',tableName,'` ADD COLUMN `',columnName,'` ',discription, ' NULL');
             end if;
         end if;

         if position != '' then #判断是否需要插入某个字段的后面
             set @stmt = CONCAT(@stmt, ' after ', position, ';');
         else
             set @stmt = CONCAT(@stmt, ';');
         end if;

         PREPARE stmt1 FROM @stmt;
         EXECUTE stmt1;
         DEALLOCATE PREPARE stmt1;
         select @stmt;
     end if;
 end$$
 delimiter ;

三、测试

调用方式：call AddColumnIfNotExists(表名, 字段名,属性, 插入到那个字段之后);

1.新建一个员工表

#员工表
CREATE TABLE emp(
    empno MEDIUMINT UNSIGNED  NOT NULL  DEFAULT 0 comment '编号',
    ename VARCHAR(20) NOT NULL DEFAULT "" comment '名字',
    job VARCHAR(9) NOT NULL DEFAULT "" comment '工作',
    mgr MEDIUMINT UNSIGNED NOT NULL DEFAULT 0 comment '上级编号'
)ENGINE=MyISAM DEFAULT CHARSET=utf8 ;

mysql> desc emp;
+-------+-----------------------+------+-----+---------+-------+
| Field | Type                  | Null | Key | Default | Extra |
+-------+-----------------------+------+-----+---------+-------+
| empno | mediumint(8) unsigned | NO   |     | 0       |       |
| ename | varchar(20)           | NO   |     |         |       |
| job   | varchar(9)            | NO   |     |         |       |
| mgr   | mediumint(8) unsigned | NO   |     | 0       |       |
+-------+-----------------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

2.新插入字段

(1)无null、无default

mysql> call AddColumnIfNotExists('emp', 'hiredatea', 'DATE comment "入职时间"','');
+-------------------------------------------------------------------------+
| @stmt                                                                   |
+-------------------------------------------------------------------------+
|  ALTER TABLE `emp` ADD COLUMN `hiredatea` DATE comment "入职时间" NULL;
|
+-------------------------------------------------------------------------+
1 row in set (0.09 sec)

Query OK, 0 rows affected (0.13 sec)

mysql> desc emp;
+-----------+-----------------------+------+-----+---------+-------+
| Field     | Type                  | Null | Key | Default | Extra |
+-----------+-----------------------+------+-----+---------+-------+
| empno     | mediumint(8) unsigned | NO   |     | 0       |       |
| ename     | varchar(20)           | NO   |     |         |       |
| job       | varchar(9)            | NO   |     |         |       |
| mgr       | mediumint(8) unsigned | NO   |     | 0       |       |
| hiredatea | date                  | YES  |     | NULL    |       |
+-----------+-----------------------+------+-----+---------+-------+
5 rows in set (0.01 sec)

(2).有not null、无default

mysql> call AddColumnIfNotExists('emp', 'sal', 'DECIMAL(7,2) not null comment "
薪水"', '');
+---------------------------------------------------------------------------+
| @stmt                                                                     |
+---------------------------------------------------------------------------+
|  ALTER TABLE `emp` ADD COLUMN `sal` DECIMAL(7,2) not null comment "薪水";   |
+---------------------------------------------------------------------------+
1 row in set (0.06 sec)

Query OK, 0 rows affected (0.09 sec)

mysql> desc emp;
+-----------+-----------------------+------+-----+---------+-------+
| Field     | Type                  | Null | Key | Default | Extra |
+-----------+-----------------------+------+-----+---------+-------+
| empno     | mediumint(8) unsigned | NO   |     | 0       |       |
| ename     | varchar(20)           | NO   |     |         |       |
| job       | varchar(9)            | NO   |     |         |       |
| mgr       | mediumint(8) unsigned | NO   |     | 0       |       |
| hiredatea | date                  | YES  |     | NULL    |       |
| sal       | decimal(7,2)          | NO   |     | NULL    |       |
+-----------+-----------------------+------+-----+---------+-------+
6 rows in set (0.01 sec)

(3).无null、有default

mysql> call AddColumnIfNotExists('emp', 'comm', 'DECIMAL(7,2) default 0 comment
"红利"', '');
+--------------------------------------------------------------------------------------+
| @stmt|
+--------------------------------------------------------------------------------------+
|  ALTER TABLE `emp` ADD COLUMN `comm` DECIMAL(7,2) default 0 comment "红利" NOT NULL;  |
+--------------------------------------------------------------------------------------+
1 row in set (0.06 sec)

Query OK, 0 rows affected (0.09 sec)

mysql> desc emp;
+-----------+-----------------------+------+-----+---------+-------+
| Field     | Type                  | Null | Key | Default | Extra |
+-----------+-----------------------+------+-----+---------+-------+
| empno     | mediumint(8) unsigned | NO   |     | 0       |       |
| ename     | varchar(20)           | NO   |     |         |       |
| job       | varchar(9)            | NO   |     |         |       |
| mgr       | mediumint(8) unsigned | NO   |     | 0       |       |
| hiredatea | date                  | YES  |     | NULL    |       |
| sal       | decimal(7,2)          | NO   |     | NULL    |       |
| comm      | decimal(7,2)          | NO   |     | 0.00    |       |
+-----------+-----------------------+------+-----+---------+-------+
7 rows in set (0.01 sec)

(4).有not null、有default、插入都某个字段之后

mysql> call AddColumnIfNotExists('emp', 'deptno', 'varchar(20) not null default
"" comment "部门编号"', 'job');
+------------------------------------------------------------------------------------------------------+
| @stmt  |
+------------------------------------------------------------------------------------------------------+
|  ALTER TABLE `emp` ADD COLUMN `deptno` varchar(20) not null default "" comment "部门编号" after job;  |
+------------------------------------------------------------------------------------------------------+
1 row in set (0.06 sec)

Query OK, 0 rows affected (0.13 sec)

mysql> desc emp;
+-----------+-----------------------+------+-----+---------+-------+
| Field     | Type                  | Null | Key | Default | Extra |
+-----------+-----------------------+------+-----+---------+-------+
| empno     | mediumint(8) unsigned | NO   |     | 0       |       |
| ename     | varchar(20)           | NO   |     |         |       |
| job       | varchar(9)            | NO   |     |         |       |
| deptno    | varchar(20)           | NO   |     |         |       |
| mgr       | mediumint(8) unsigned | NO   |     | 0       |       |
| hiredatea | date                  | YES  |     | NULL    |       |
| sal       | decimal(7,2)          | NO   |     | NULL    |       |
| comm      | decimal(7,2)          | NO   |     | 0.00    |       |
+-----------+-----------------------+------+-----+---------+-------+
8 rows in set (0.01 sec)