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MySQL-5.7 高阶语法及流程控制

时间:2021-07-01 10:21:17 帮助过:13人阅读

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)

MySQL-5.7 高阶语法及流程控制

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