MySQL-5.7 高阶语法及流程控制

1.标签语句

[begin_label:] BEGIN
    [statement_list]
END [end_label]
[begin_label:] LOOP
    statement_list
END LOOP [end_label]
[begin_label:] REPEAT
    statement_list
UNTIL search_condition
END REPEAT [end_label]
[begin_label:] WHILE search_condition DO
    statement_list
END WHILE [end_label]

• 标签label可以加在begin...end语句以及loop，repeat和while语句上
• 语句中通过iterate和leave来控制流程，iterate表示返回指定标签位置，leave表示跳出标签

实例：

mysql> delimiter //
mysql> create procedure doiterate(in p1 int,out p2 int)
    -> begin
    -> label1:loop
    -> set p1 = p1 + 1;
    -> if p1 < 10 then iterate label1;end if;
    -> leave label1;
    -> end loop label1;
    -> set p2=p1;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql> call doiterate(1,@a);
Query OK, 0 rows affected (0.00 sec)
mysql> select @a;
+------+
| @a   |
+------+
|   10 |
+------+
1 row in set (0.00 sec)
mysql> call doiterate(5,@a);
Query OK, 0 rows affected (0.00 sec)
mysql> select @a;
+------+
| @a   |
+------+
|   10 |
+------+
1 row in set (0.00 sec)

2.Declare语句

语法：

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

• Declare语句通常声明本地变量、游标、条件或者handler
• Declare语句只允许出现在begin...end语句中而且必须出现在第一行
• Declare的顺序也有要求，通常是先声明本地变量，再是游标，然后是条件和handler
• 使用default指定变量的默认值，如果没有指定默认值则为NULL
• 声明的变量和被引用的数据表中的字段要区分开

存储过程中的变量

• 本地变量可以通过declare语句声明
• 声明后的变量可以通过select...into var_list进行赋值，或者通过set语句赋值，或者通过定义游标并使用fetch...into var_list赋值

mysql> delimiter //
mysql> create procedure sp1(v_sid int)
    -> begin
    -> declare xname varchar(64) default 'bob';
    -> declare xgender int;
    -> select sname,gender into xname,xgender from students where sid=v_sid;
    -> select xname,xgender;
    -> end//
Query OK, 0 rows affected (0.01 sec)
mysql> delimiter ;
mysql>
mysql> call sp1(1);
+--------+---------+
| xname  | xgender |
+--------+---------+
| Andrew |       1 |
+--------+---------+
1 row in set (0.00 sec)
Query OK, 0 rows affected (0.00 sec)

3.流程控制语句

（1）case语句

在存储过程或函数中表明复杂的条件选择语句。

语法：

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

说明：

• case_value与when_value依次做相等对比，如果相等则执行对应的后面的SQL语句，否则接着对比；
• 当search_condition满足true/1的结果时，则执行对应的SQL语句，否则执行else对应的SQL语句；

实例：

mysql> delimiter //
mysql> create procedure exp_case(v_sid int)
    -> begin
    -> declare v int default 1;
    -> select gender into v from students where sid=v_sid;
    -> case v
    -> when 0 then update students set gender=1 where sid=v_sid;
    -> when 1 then update students set gender=0 where sid=v_sid;
    -> else
    -> update students set gender=-1 where sid=v_sid;
    -> end case;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql>
mysql> select * from students where sid=1;
+-----+--------+--------+---------+
| sid | sname  | gender | dept_id |
+-----+--------+--------+---------+
|   1 | Andrew | 1      |       1 |
+-----+--------+--------+---------+
1 row in set (0.00 sec)
mysql> call exp_case(1);
Query OK, 1 row affected (0.03 sec)
mysql> select * from students where sid=1;
+-----+--------+--------+---------+
| sid | sname  | gender | dept_id |
+-----+--------+--------+---------+
|   1 | Andrew | 0      |       1 |
+-----+--------+--------+---------+
1 row in set (0.00 sec)
另一种写法：
mysql> delimiter //
mysql> create procedure exp_case2(v_sid int)
    -> begin
    -> declare v int default 1;
    -> select gender into v from students where sid=v_sid;
    -> case
    -> when v=0 then update students set gender=1 where sid=v_sid;
    -> when v=1 then update students set gender=0 where sid=v_sid;
    -> else
    -> update students set gender=-1 where sid=v_sid;
    -> end case;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql> call exp_case2(1);
Query OK, 1 row affected (0.03 sec)
mysql> select * from students where sid=1;
+-----+--------+--------+---------+
| sid | sname  | gender | dept_id |
+-----+--------+--------+---------+
|   1 | Andrew | 1      |       1 |
+-----+--------+--------+---------+
1 row in set (0.00 sec)

（2）IF语句

在存储过程或函数中表明基础的条件选择语句

语法：

IF search_condition THEN statement_list
    [ELSEIF search_condition THEN statement_list] ...
    [ELSE statement_list]
END IF

• 如果search_condition满足true/1的条件，则执行对应的SQL语句，否则再判断elseif中的search_condition，都不满足则执行else中的SQL语句；
• statement_list中可以包含一个或多个SQL语句

实例：

mysql> delimiter //
mysql> 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//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql>
mysql> select SimpleCompare(5,6);
+--------------------+
| SimpleCompare(5,6) |
+--------------------+
| 5<6                |
+--------------------+
1 row in set (0.00 sec)
mysql> select SimpleCompare(15,34);
+----------------------+
| SimpleCompare(15,34) |
+----------------------+
| 15<34                |
+----------------------+
1 row in set (0.00 sec)
mysql> select SimpleCompare(78,78);
+----------------------+
| SimpleCompare(78,78) |
+----------------------+
| 78=78                |
+----------------------+
1 row in set (0.00 sec)

IF嵌套：

mysql> delimiter //
mysql> create function verboseCompare(n int,m int)
    -> returns varchar(50)
    -> begin
    -> declare s varchar(50);
    -> if n = m then set s = 'equals';
    -> else
    -> if n > m then set s = 'greater';
    -> else set s = 'less';
    -> end if;
    -> set s = concat('is',s,'than');
    -> end if;
    -> set s = concat(n,'',s,'',m,'.');
    -> return s;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql> select verboseCompare(4,5);
+---------------------+
| verboseCompare(4,5) |
+---------------------+
| 4islessthan5.       |
+---------------------+
1 row in set (0.00 sec)

（3）iterate语句

仅出现在loop，repeat，while循环语句中，表示重新开始此循环。

语法：

ITERATE label

（4）leave语句

表明指定标签的流程控制语句块，通常用在begin...end，以及loop，repeat，while循环汇总；

LEAVE label

实例：

mysql> delimiter //
mysql> create procedure doiterate2(in p1 int,out p2 int)
    -> begin
    -> label1:loop
    -> set p1=p1+1;
    -> if p1 < 10 then iterate label1;end if;
    -> leave label1;
    -> end loop label1;
    -> set p2=p1;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;

（5）loop语句

在存储过程或函数中表达循环执行的一种方式；

语法：

[begin_label:] LOOP
    statement_list
END LOOP [end_label]

实例：

mysql> delimiter //
mysql> create procedure doiterate3(p1 int)
    -> begin
    -> label1:loop
    -> set p1=p1+1;
    -> if p1<10 then iterate label1;end if;
    -> leave label1;
    -> end loop label1;
    -> set @x=p1;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;

（6）repeat语句

在存储过程或函数中表达循环执行的一种方式；

语法：

[begin_label:] REPEAT
    statement_list
UNTIL search_condition
END REPEAT [end_label]

实例：

mysql> delimiter //
mysql> create procedure doiterate4(p1 int)
    -> begin
    -> set @x=0;
    -> repeat
    -> set @x=@x+1;
    -> until @x>p1 end repeat;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql> call doiterate4(1000);
Query OK, 0 rows affected (0.02 sec)
mysql> select @x;
+------+
| @x   |
+------+
| 1001 |
+------+
1 row in set (0.00 sec)

（7）while语句

在存储过程或函数中表达循环执行的一种方式；

语法：

[begin_label:] WHILE search_condition DO
    statement_list
END WHILE [end_label]

• 当search_condition返回true时，循环执行SQL语句，直到search_condition为false；

实例：

mysql> delimiter //
mysql> create procedure dowhile()
    -> begin
    -> declare v1 int default 5;
    -> while v1>0 do
    -> update students set gender=-1 where sid=v1;
    -> set v1=v1-1;
    -> end while;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql> call dowhile();
Query OK, 1 row affected (0.63 sec)
mysql> select * from students;
+-----+--------+--------+---------+
| sid | sname  | gender | dept_id |
+-----+--------+--------+---------+
|   1 | Andrew | -1     |       1 |
|   2 | Andy   | -1     |       1 |
|   3 | Bob    | -1     |       1 |
|   4 | Ruth   | -1     |       2 |
|   5 | Mike   | -1     |       2 |
|   6 | John   | 0      |       3 |
|   7 | Cindy  | 1      |       3 |
|   8 | Susan  | 1      |       3 |
+-----+--------+--------+---------+
8 rows in set (0.00 sec)

（8）return语句

在函数中，用来终结函数的执行并将指定值返回给调用者；

语法：

RETURN expr

• 在函数中必须有至少一个return语句，当有多个return语句时则表明函数有多种退出方式；

实例：

mysql> delimiter //
mysql> create function doreturn()
    -> returns int
    -> begin
    -> select gender into @a from students where sid = 1;
    -> if @a=1 then return 1;
    -> elseif @a=0 then return 0;
    -> else return 999;
    -> end if;
    -> end//
Query OK, 0 rows affected (0.00 sec)
mysql> delimiter ;
mysql> select doreturn();
+------------+
| doreturn() |
+------------+
|        999 |
+------------+
1 row in set (0.00 sec)
mysql> select * from students where sid = 1;
+-----+--------+--------+---------+
| sid | sname  | gender | dept_id |
+-----+--------+--------+---------+
|   1 | Andrew | -1     |       1 |
+-----+--------+--------+---------+
1 row in set (0.00 sec)