mysql 存储过程，函数，触发器

DECLARE var_name [, var_name] ... type [DEFAULT value]

使用default指定变量的默认值，如果没有指定默认值则初始值为NULL
Type指明该变量的数据类型
声明的变量作用范围为被声明的begin … end语句块之间
声明的变量和被引用的数据表中的字段名要区分开来

delimiter //
create procedure sp1(s_sid int)
comment ‘查询该s_sid的姓名和性别,声明了两个变量xname,xgender‘
begin 
declare xname varchar(8) default ‘xixi‘;
declare xgender int;
select sname,gender into xname,xgender
from students where sid=s_sid;
select xname,xgender;
end ;
//
delimiter ;
mysql> call sp1(2);
+-------+---------+
| xname | xgender |
+-------+---------+
| Andy  |       0 |
+-------+---------+
1 row in set (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

mysql> HELP CASE STATEMENT;
Name: ‘CASE STATEMENT‘
Description:
Syntax:
CASE case_value
    WHEN when_value THEN statement_list
    [WHEN when_value THEN statement_list] ...
    [ELSE statement_list]
END CASE

Or:

CASE
    WHEN search_condition THEN statement_list
    [WHEN search_condition THEN statement_list] ...
    [ELSE statement_list]
END CASE

第一个语句中case_value与后面各句的when_value依次做相等的对比， 如果碰到相等的，则执行对应的后面的statement_list，否则接着对比，如果都没有匹配，则执行else后面的statement_list

第二个语句中当search_condition满足true/1的结果时，则执行对应的statement_list，否则执行else对应的statement_list

Statement_list可以包含一个或多个SQL语句

DELIMITER |

CREATE PROCEDURE p()
  BEGIN
    DECLARE v INT DEFAULT 1;

    CASE v
      WHEN 2 THEN SELECT v;
      WHEN 3 THEN SELECT 0;
      ELSE
        BEGIN
        END;
    END CASE;
  END;
  |
DELIMTER ;

mysql> help if statement;
Name: ‘IF STATEMENT‘
Description:
Syntax:
IF search_condition THEN statement_list
    [ELSEIF search_condition THEN statement_list] ...
    [ELSE statement_list]
END IF
IF语句中如果search_condition满足true/1的条件，则执行对应的statement_list，否则再判断elseif中的search_condition是否满足
true/1的条件，如果都不满足则执行else中的statement_list语句

Statement_list中可以包含一个或多个SQL语句

DELIMITER //
CREATE FUNCTION SimpleCompare(n INT, m INT)
RETURNS VARCHAR(20)
BEGIN
DECLARE s VARCHAR(20);
IF n > m THEN SET s = ‘>‘;
ELSEIF n = m THEN SET s = ‘=‘;
ELSE SET s = ‘<‘;
END IF;
SET s = CONCAT(n, ‘ ‘, s, ‘ ‘, m);
RETURN s;
END //

DELIMITER ;
mysql> select simplecompare(1,2);
+--------------------+
| simplecompare(1,2) |
+--------------------+
| 1 < 2              |
+--------------------+

mysql> help iterate
Name: ‘ITERATE‘
Description:
Syntax:
ITERATE label
Label表示自定义的标签名

mysql> help leave
Name: ‘LEAVE‘
Description:
Syntax:
LEAVE label

Label表明要退出的标签名

mysql> HELP LOOP
Name: ‘LOOP‘
Description:
Syntax:
[begin_label:] LOOP
    statement_list
END LOOP [end_label]

mysql> HELP REPEAT
Name: ‘REPEAT LOOP‘
Description:
Syntax:
[begin_label:] REPEAT
    statement_list
UNTIL search_condition
END REPEAT [end_label]

mysql> help while
Name: ‘WHILE‘
Description:
Syntax:
[begin_label:] WHILE search_condition DO
    statement_list
END WHILE [end_label]
• 当search_condition返回为true时，则循环执行statement_list中的语句，直到
search_condition的结果返回为false
• Statement_list中可以包含一个或多个SQL语句

mysql> HELP RETURN
Name: ‘RETURN‘
Description:
Syntax:
RETURN expr
• 在函数中必须要有至少一个return语句，当有多个return语句时则表明函数多种退出的方式

mysql> help  close;
Name: ‘CLOSE‘
Description:
Syntax:
CLOSE cursor_name

如果关闭一个未打开的游标，则MySQL会报错
如果在存储过程和函数中未使用此语句关闭已经打开的游标，则游标会在声明的begin…end语句块执行完之后自动关闭

mysql> help cursor;
Name: ‘DECLARE CURSOR‘
Description:
Syntax:
DECLARE cursor_name CURSOR FOR select_statement

• Select_statement代表一个select语句

mysql> help fetch;
Name: ‘FETCH‘
Description:
Syntax:
FETCH [[NEXT] FROM] cursor_name INTO var_name [, var_name] ...

数据集中的字段需要和INTO语句中定义的变量一一对应

数据集中的数据都fetch完之后，则返回NOT FOUND

mysql> HELP OPEN
Name: ‘OPEN‘
Description:
Syntax:
OPEN cursor_name

mysql> help declare condition
Name: ‘DECLARE CONDITION‘
Description:
Syntax:
DECLARE condition_name CONDITION FOR condition_value

condition_value:
    mysql_error_code
  | SQLSTATE [VALUE] sqlstate_value

Condition_value指定特定的错误条件，可以有以下两种形式
• Mysql_err_code表示MySQL error code的整数
• SQLSTATE sqlstate_value表示MySQL中用5位字符串表达的语句状态
mysql> insert into students (sid) values (1);
ERROR 1062 (23000): Duplicate entry ‘1‘ for key ‘PRIMARY‘
23000 就是sqlstate_value 主键错误
1062表示mysql_err_code

比如在MySQL中1051error code表示的是unknown table的错误，如果要对这
个错误做特殊处理，可以用三种种方法：


1. DECLARE CONTINUE HANDLER FOR 1051
    BEGIN
    -- body of handler
    END;

2. DECLARE no_such_table CONDITION FOR 1051;
   DECLARE CONTINUE HANDLER FOR no_such_table
   BEGIN
   -- body of handler
   END;

3. DECLARE no_such_table CONDITION FOR SQLSTATE ‘42S02‘;
    DECLARE CONTINUE HANDLER FOR no_such_table
    BEGIN
     -- body of handler
    END;

mysql> help declare handler;
Name: ‘DECLARE HANDLER‘
Description:
Syntax:
DECLARE handler_action HANDLER
    FOR condition_value [, condition_value] ...
    statement

handler_action:
    CONTINUE
  | EXIT
  | UNDO

condition_value:
    mysql_error_code
  | SQLSTATE [VALUE] sqlstate_value
  | condition_name
  | SQLWARNING
  | NOT FOUND
  | SQLEXCEPTION

Handler_action子句声明当执行完statement语句之后应该怎么办
    Continue代表继续执行该存储过程或函数
    Exit代表退出声明此handler的begin…end语句块
    Undo参数已经不支持 mysql 5.7
Condition_value的值有以下几种：
     Mysql_err_code表示MySQL error code的整数
     SQLSTATE sqlstate_value表示MySQL中用5位字符串表达的语句状态
     Condition_name表示之前在declare…condition语句中声明的名字
     SQLWARNING表示所有的警告信息，即SQLSTATE中01打头的所有错误
     NOT FOUND表示查完或者查不到数据，即SQLSTATE中02打头的所有错误
     SQLEXCEPTION表示所有的错误信息

当condition发生但没有声明handler时，则存储过程和函数依照如
下规则处理
    发生SQLEXCEPTION错误，则执行exit退出
    发生SQLWARNING警告，则执行contine继续执行
    发生NOT FOUND情况，则执行continue继续执行

• DECLARE CONTINUE HANDLER FOR 1051
• BEGIN
• -- body of handler
• END;
• •
DECLARE CONTINUE HANDLER FOR SQLSTATE ‘42S02‘
• BEGIN
• -- body of handler
• END;
• •
DECLARE CONTINUE HANDLER FOR SQLWARNING
• BEGIN
• -- body of handler
• END;
• •
DECLARE CONTINUE HANDLER FOR SQLEXCEPTION
• BEGIN
• -- body of handler
• END; 

• mysql> CREATE TABLE t (s1 INT, PRIMARY KEY (s1));
• Query OK, 0 rows affected (0.00 sec)
delimiter //
CREATE PROCEDURE handlerdemo ()
BEGIN
DECLARE CONTINUE HANDLER FOR SQLSTATE ‘23000‘ SET @x2 = 1;
SET @x = 1;
INSERT INTO test.t VALUES (1);
SET @x = 2;
INSERT INTO test.t VALUES (1);
SET @x = 3;
END;
//
delimiter ;

#当@x = 1  已近往test.t 表下往主键写入了1 
#当@x = 1  再往test.t 写入1 这时候就触发了declare hander  然后不执行哪个主键冲突语句， 执行set @x=3

然后通过select @x; 查看变量该值是3

>Cursor游标用来声明一个`数据集`
>游标的声明必须在变量和条件声明之后，在handler声明之前

 CREATE PROCEDURE curdemo()
 BEGIN
 DECLARE done INT DEFAULT FALSE;
 DECLARE a CHAR(16);
 DECLARE b, c INT;
 DECLARE cur1 CURSOR FOR SELECT id,data FROM test.t1;
 DECLARE cur2 CURSOR FOR SELECT i FROM test.t2;
 DECLARE CONTINUE HANDLER FOR NOT FOUND SET done = TRUE;
 OPEN cur1;
 OPEN cur2;
 read_loop: LOOP
 FETCH cur1 INTO a, b;
 FETCH cur2 INTO c;
 IF done THEN
 LEAVE read_loop;
 END IF;
 IF b < c THEN
 INSERT INTO test.t3 VALUES (a,b);
 ELSE
 INSERT INTO test.t3 VALUES (a,c);• END IF;
 END LOOP;
 CLOSE cur1;
 CLOSE cur2;
 END;

mysql> HELP CREATE TRIGGER;
Name: ‘CREATE TRIGGER‘
Description:
Syntax:
CREATE
    [DEFINER = { user | CURRENT_USER }]
    TRIGGER trigger_name
    trigger_time trigger_event
    ON tbl_name FOR EACH ROW
    [trigger_order]
    trigger_body

trigger_time: { BEFORE | AFTER }

trigger_event: { INSERT | UPDATE | DELETE }

trigger_order: { FOLLOWS | PRECEDES } other_trigger_name


Definer关键词用来指定trigger的安全环境

Trigger_time指定触发器的执行时间， BEFORE和AFTER指定触发器在表中的每行数据修改前或者后执行
Trigger_event指定触发该触发器的具体事件
    • INSERT当新的一行数据插入表中时触发，比如通过执行insert,loaddata,replace语句插入新数据
    • UPDATE当表的一行数据被修改时触发，比如执行update语句时
    • DELETE当表的一行数据被删除时触发，比如执行delete,replace语句时

当执行insert into … on duplicate key update语句时，当碰到重复行执行update时，则触发update下的触发器  

从5.7.2版本开始，可以创建具有相同trigger_time和trigger_event的同一个表上的多个触发器，默认情况下按照创建的时间依次执行，通过指定FOLLOWS/PRECEDES改变执行顺序，即FOLLOWS时表示新创建的触发器后执PRECEDES则表示新触发器先执行

Trigger_body表示触发器触发之后要执行的一个或多个语句，在内部可以引用涉及表的字OLD.col_name表示行数据被修改或删除之前的字段数据， NEW.col_name表示行数据被插入或修改之后的字段数据

mysql> create table students_bak
    ->  (old_sid int , old_sname varchar(64),new_sname varchar(64) , old_gender int , 
    ->  new_gender int ,tstamp datetime);
Query OK, 0 rows affected (0.00 sec)

mysql> 
mysql> 
mysql> desc students_bak;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| old_sid    | int(11)     | YES  |     | NULL    |       |
| old_sname  | varchar(64) | YES  |     | NULL    |       |
| new_sname  | varchar(64) | YES  |     | NULL    |       |
| old_gender | int(11)     | YES  |     | NULL    |       |
| new_gender | int(11)     | YES  |     | NULL    |       |
| tstamp     | datetime    | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
mysql> desc students;
+---------+-------------+------+-----+---------+----------------+
| Field   | Type        | Null | Key | Default | Extra          |
+---------+-------------+------+-----+---------+----------------+
| sid     | int(11)     | NO   | PRI | NULL    | auto_increment |
| sname   | varchar(64) | YES  |     | NULL    |                |
| gender  | varchar(12) | YES  |     | NULL    |                |
| dept_id | int(11)     | YES  | MUL | NULL    |                |
+---------+-------------+------+-----+---------+----------------+

#创建一个触发器
delimiter //
create trigger simple_trigger
after update
on students for each row
begin
insert into students_bak values(old.sid,old.sname,new.sname,old.gender,new.gender,now());
end;
//
delimiter ;

#
mysql> select * from students;
+-----+--------+--------+---------+
| sid | sname  | gender | dept_id |
+-----+--------+--------+---------+
|   1 | Andrew | 0      |       1 |
|   2 | Andy   | 0      |       1 |
|   3 | Bob    | 0      |       1 |
|   4 | Ruth   | 1      |       2 |
|   5 | Mike   | 0      |       2 |
|   6 | John   | 0      |       3 |
|   7 | Cindy  | 1      |       3 |
|   8 | Susan  | 1      |       3 |
+-----+--------+--------+---------+
8 rows in set (0.00 sec)

mysql> select * from students_bak;
Empty set (0.00 sec)

#执行update 操作，会自动触发器的运行
mysql> update students set sname=‘abc‘,gender=1;
Query OK, 8 rows affected (0.00 sec)
Rows matched: 8  Changed: 8  Warnings: 0

mysql> select * from students_bak;
+---------+-----------+-----------+------------+------------+---------------------+
| old_sid | old_sname | new_sname | old_gender | new_gender | tstamp              |
+---------+-----------+-----------+------------+------------+---------------------+
|       1 | Andrew    | abc       |          0 |          1 | 2018-07-31 22:21:39 |
|       2 | Andy      | abc       |          0 |          1 | 2018-07-31 22:21:39 |
|       3 | Bob       | abc       |          0 |          1 | 2018-07-31 22:21:39 |
|       4 | Ruth      | abc       |          1 |          1 | 2018-07-31 22:21:39 |
|       5 | Mike      | abc       |          0 |          1 | 2018-07-31 22:21:39 |
|       6 | John      | abc       |          0 |          1 | 2018-07-31 22:21:39 |
|       7 | Cindy     | abc       |          1 |          1 | 2018-07-31 22:21:39 |
|       8 | Susan     | abc       |          1 |          1 | 2018-07-31 22:21:39 |
+---------+-----------+-----------+------------+------------+---------------------+
8 rows in set (0.00 sec)

mysql> select * from students;
+-----+-------+--------+---------+
| sid | sname | gender | dept_id |
+-----+-------+--------+---------+
|   1 | abc   | 1      |       1 |
|   2 | abc   | 1      |       1 |
|   3 | abc   | 1      |       1 |
|   4 | abc   | 1      |       2 |
|   5 | abc   | 1      |       2 |
|   6 | abc   | 1      |       3 |
|   7 | abc   | 1      |       3 |
|   8 | abc   | 1      |       3 |
+-----+-------+--------+---------+
8 rows in set (0.00 sec)

mysql> select trigger_schema,trigger_name from information_schema.triggers;
+----------------+----------------------------+
| trigger_schema | trigger_name               |
+----------------+----------------------------+
| course         | simple_trigger             |
| sys            | sys_config_insert_set_user |
| sys            | sys_config_update_set_user |
+----------------+----------------------------+
3 rows in set (0.00 sec)

trigger_schema： 哪个库
trigger_name ： 触发器的名字

sys 库下面的触发器是默认就有的

查看触发器的详细信息
show create trigger course.simple_trigger

mysql> help drop trigger;
Name: ‘DROP TRIGGER‘
Description:
Syntax:
DROP TRIGGER [IF EXISTS] [schema_name.]trigger_name

• If exists短语用来避免删除不存在的触发器时引发报错
• 当你执行drop table时，表上的触发器也被drop掉了