MySQL learning talks about process control and cursors-Mysql Tutorial-php.cn

This article is about learning MySQL and talking about process control and cursors in MySQL. I hope it will be helpful to everyone.

MySQL learning talks about process control and cursors

Anyone who has studied or understood programming languages knows that any programming language cannot be completed with just one or two statements of code.

The function of flow control statements is to control the execution order of statements during the running of the program. It is an indispensable part for us to complete complex operations.

As long as it is an executed program, the process is divided into three categories:

Sequential structure: The program is executed sequentially from top to bottom
Branch structure: The program is selected and executed according to conditions, and one of two or more paths is selected for execution.
Loop structure: When the program meets certain conditions, it is repeated Execute a set of statements

There are three main types of flow control statements for MySQL. Note: can only be used for stored procedures.

Conditional judgment statement: IF statement and CASE statement
Loop statement: LOOP, WHILE and REPEAT statement
Jump statement: ITERATE and LEAVE statements

Preparation work:

Create the database and two table, and insert data:

create database dbtest16;
use dbtest16;

create table employees 
as 
select * from atguigudb.employees;
create table departments
as
select * from atguigudb.departments;

select * from employees;
select * from departments;

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MySQL learning talks about process control and cursors

##Branch structure

Branch, that is, choose one of the two. There are two main forms of presentation of branch structures in SQL:

IF statement

IF 表达式1 THEN 操作1
[ELSEIF 表达式2 THEN 操作2]……
[ELSE 操作N]
END IF

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The corresponding statement is executed according to whether the result of the expression is TRUE or FALSE. The content in "[]" here is optional.

Example 1:

Single judgment

delimiter //
create procedure test_if()
begin
		# 声明局部变量
		declare stu_name varchar(15);
		if stu_name is null
							then select &#39;stu_name is null&#39;;
		end if;
end//
delimiter ;
call test_if();

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MySQL learning talks about process control and cursors

Example two:

Choose one from two

delimiter //
create procedure test_if2()
begin
		declare email varchar(25);
		
		if email is null
					then select &#39;email is null&#39;;
		else
					select &#39;email is not null&#39;;
		end if;
end//
delimiter ;
call test_if2();

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MySQL learning talks about process control and cursors

Example three:

Choose one from more

delimiter //
create procedure test_if3()
begin
		declare age int default 20;
		
		if age > 40
					then select &#39;中老年&#39;;
		elseif age > 18
					then select &#39;青壮年&#39;;
		elseif age > 10
					then select &#39;青少年&#39;;
		else 
					select &#39;孩童&#39;;
		end if;
end//
delimiter ;
call test_if3();

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MySQL learning talks about process control and cursors

Scenario Example 1: Declare the stored procedure "update_salary_by_eid1", define the IN parameter emp_id, and enter the employee number. If it is judged that the employee's salary is less than 8,000 yuan and the employee has been employed for more than 5 years, the salary will be increased by 500 yuan; otherwise, it will remain unchanged.

delimiter //
create procedure update_salary_by_eid1(in emp_id int)
begin
  # 声明变量 
	declare sal double;  # 记录员工工资
	declare hiredate date;  # 记录入职日期
	# 查询赋值
	select salary,hire_date into sal,hiredate from employees 
	where employee_id = emp_id;
	# 判断条件并修改
	if sal < 8000 and datediff(now(),hiredate)/365 > 5 
				then update employees set salary = salary + 500 where employee_id = emp_id;
	end if;
end//
delimiter;

set @emp_id = 104;
# 更新前104号员工工资情况：
select salary,employee_id from employees where employee_id = @emp_id;
# 更新工资：
call update_salary_by_eid1(@emp_id);
# 再次查询104号员工工资情况：
select salary,employee_id from employees where employee_id = @emp_id;

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MySQL learning talks about process control and cursors

Scenario Example 2: Declare the stored procedure "update_salary_by_eid2", define the IN parameter emp_id, and enter the employee number . It is judged that if the employee's salary is less than 9,000 yuan and the employee has been employed for more than 5 years, the salary will be increased by 500 yuan; otherwise, the salary will be increased by 100 yuan.

delimiter //
create procedure update_salary_by_eid2(in emp_id int)
begin

  # 声明变量 

	declare sal double;  # 记录员工工资
	declare hiredate date;  # 记录入职日期
	# 查询赋值
	select salary,hire_date into sal,hiredate from employees 
	where employee_id = emp_id;
	# 判断条件并修改
	if sal < 9000 and datediff(now(),hiredate)/365 > 5 
				then update employees set salary = salary + 500 where employee_id = emp_id;
	else 
				update employees set salary = salary + 100 where employee_id = emp_id;
	end if;

end//
delimiter;
# 以103,104员工为例
# 更新前员工工资情况：
select salary,employee_id from employees where employee_id in (103,104);
# 更新工资：
call update_salary_by_eid2(103);
call update_salary_by_eid2(104);
# 再次查询员工工资情况：
select salary,employee_id from employees where employee_id in (103,104);

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MySQL learning talks about process control and cursors

Scenario example three: Declare the stored procedure "update_salary_by_eid3", define the IN parameter emp_id, and enter the employee number . If it is judged that the employee's salary is less than 9,000 yuan, the salary will be updated to 9,000 yuan; if the salary is greater than or equal to 9,000 yuan and less than 10,000, but the bonus ratio is NULL, the bonus ratio will be updated to 0.01; other salary increases will be 100 yuan.

delimiter //
create procedure update_salary_by_eid3(in emp_id int)
begin
  # 声明变量 
	declare sal double;  # 记录员工工资
	declare emp_commission_pct double;  # 记录奖金比例
	# 查询赋值
	select salary,commission_pct into sal,emp_commission_pct from employees 
	where employee_id = emp_id;
	# 判断条件并修改
	if sal < 9000
			then update employees set salary = 9000 where employee_id = emp_id;
	elseif sal < 10000 and emp_commission_pct is null
	    then update employees set commission_pct = 0.01 where employee_id = emp_id;
	else
			update employees set salary = salary + 100 where employee_id = emp_id;
	end if;
end//
delimiter;

# 以102,103,104员工为例
# 更新前员工工资情况：
select salary,employee_id,commission_pct from employees where employee_id in (102,103,104);
# 更新工资：
call update_salary_by_eid3(102);
call update_salary_by_eid3(103);
call update_salary_by_eid3(104);
# 再次查询员工工资情况：
select salary,employee_id,commission_pct from employees where employee_id in (102,103,104);

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MySQL learning talks about process control and cursors

1MySQL learning talks about process control and cursors

CASE statement

#情况一：类似于switch
CASE 表达式
WHEN 值1 THEN 结果1或语句1(如果是语句，需要加分号) 
WHEN 值2 THEN 结果2或语句2(如果是语句，需要加分号)
...
ELSE 结果n或语句n(如果是语句，需要加分号)
END [case]（如果是放在begin end中需要加上case，如果放在select后面不需要）

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#情况二：类似于多重if
CASE 
WHEN 条件1 THEN 结果1或语句1(如果是语句，需要加分号) 
WHEN 条件2 THEN 结果2或语句2(如果是语句，需要加分号)
...
ELSE 结果n或语句n(如果是语句，需要加分号)
END [case]（如果是放在begin end中需要加上case，如果放在select后面不需要）

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Example 1:

case ... when ... then ...

delimiter //
create procedure test_case()
begin
		declare var int default 2;
		
		case var
			    when 1 then select &#39;var = 1&#39;;
					when 2 then select &#39;var = 2&#39;;
					when 3 then select &#39;var = 3&#39;;
					else select &#39;other&#39;;
		end case;
end //
delimiter ;
call test_case();

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1MySQL learning talks about process control and cursors

Example two:

case when ... then ...

delimiter //
create procedure test_case2()
begin
		declare var1 int default 10;
		
		case  when var1 >= 100 then select &#39;三位数&#39;;
					when var1 >=10 then select &#39;两位数&#39;;
					else select &#39;个位数&#39;;
		end case;
end //
delimiter ;
call test_case2();

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1MySQL learning talks about process control and cursors

Scenario example one : -- Declare the stored procedure "update_salary_by_eid4", define the IN parameter emp_id, and enter the employee number. -- If it is determined that the employee's salary is less than 9,000 yuan, update the salary to 9,000 yuan; -- If the salary is greater than or equal to 9,000 yuan and less than 10,000 yuan, but the bonus ratio is NULL, update the bonus ratio to 0.01; --Others will receive a salary increase of 100 yuan.

delimiter //
create procedure update_salary_by_eid4(in emp_id int)
begin
  # 声明变量 
	declare sal double;  # 记录员工工资
	declare emp_commission_pct double;  # 记录奖金比例
	# 查询赋值
	select salary,commission_pct into sal,emp_commission_pct from employees 
	where employee_id = emp_id;
	# 判断条件并修改
	case
	when sal < 9000
			then update employees set salary = 9000 where employee_id = emp_id;
	when sal < 10000 and emp_commission_pct is null
	    then update employees set commission_pct = 0.01 where employee_id = emp_id;
	else
			update employees set salary = salary + 100 where employee_id = emp_id;
	end case;
end//
delimiter;
# 以103,104,105员工为例
# 更新前员工工资情况：
select salary,employee_id,commission_pct from employees where employee_id in (103,104,105);
# 更新工资：
call update_salary_by_eid3(103);
call update_salary_by_eid3(104);
call update_salary_by_eid3(105);
# 再次查询员工工资情况：
select salary,employee_id,commission_pct from employees where employee_id in (103,104,105);

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1MySQL learning talks about process control and cursors

场景举例二：-- 声明存储过程update_salary_by_eid5，定义IN参数emp_id，输入员工编号。 -- 判断该员工的入职年限，如果是0年，薪资涨50； -- 如果是1年，薪资涨100； -- 如果是2年，薪资涨200； -- 如果是3年，薪资涨300； -- 如果是4年，薪资涨400； -- 其他的涨薪500。

delimiter //
create procedure update_salary_by_eid5(in emp_id int)
begin
  # 声明变量 
	declare sal double;  # 记录员工工资
	declare hire_year double;  # 记录入职日期
	# 查询赋值
	select salary,round(datediff(now(),hire_date)/365) into sal,hire_year from employees 
	where employee_id = emp_id;
	# 判断条件并修改
	case hire_year
			when 0 then update employees set salary = salary + 50 where employee_id = emp_id;
			when 1 then update employees set salary = salary + 100 where employee_id = emp_id;
			when 2 then update employees set salary = salary + 200 where employee_id = emp_id;
			when 3 then update employees set salary = salary + 300 where employee_id = emp_id;
			when 4 then update employees set salary = salary + 400 where employee_id = emp_id;
			else update employees set salary = salary + 500 where employee_id = emp_id;
	end case;
end//
delimiter;
# 以107员工为例
# 更新前员工工资情况：
select salary,employee_id,hire_date from employees where employee_id = 107;
# 更新工资：
call update_salary_by_eid5(107);
# 再次查询员工工资情况：
select salary,employee_id,hire_date from employees where employee_id = 107;

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1MySQL learning talks about process control and cursors

# 针对场景二，此种写法略显不足，重复的书写相同的更新语句，
# 其实观察下来也就金额不同，可以有改进改进如下：
delimiter //
create procedure update_salary_by_eid6(in emp_id int)
begin
  # 声明变量 
	declare sal double;  # 记录员工工资
	declare hire_year double;  # 记录入职日期
	declare add_sal double; # 保存更新的金额
	# 查询赋值
	select salary,round(datediff(now(),hire_date)/365) into sal,hire_year from employees 
	where employee_id = emp_id;
	# 判断条件并修改
	case hire_year
			when 0 then set add_sal = 50;
			when 1 then set add_sal = 100;
			when 2 then set add_sal = 200;
			when 3 then set add_sal = 300;
			when 4 then set add_sal = 400;
			else set add_sal = 500;
	end case;
	# 根据当前add_sal值修改
	update employees set salary = salary + add_sal where employee_id = emp_id;
end//
delimiter;
# 以108员工为例
# 更新前员工工资情况：
select salary,employee_id,hire_date from employees where employee_id = 108;
# 更新工资：
call update_salary_by_eid5(108);
# 再次查询员工工资情况：
select salary,employee_id,hire_date from employees where employee_id = 108;

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1MySQL learning talks about process control and cursors

循环结构

有时候我们需要重复的执行某条语句，而借助循环结构可以很好地实现。在MySQL中我们可以有三种方式实现循环：

LOOP
WHILE
REPEAT

凡是循环结构都遵循的四要素：

1.初始化条件
2.循环条件
3.循环体
4.迭代条件

LOOP语句

LOOP循环语句用来重复执行某些语句。LOOP内的语句一直重复执行直到循环被退出（使用LEAVE子句），跳出循环过程。

LOOP语句的基本格式如下：

[loop_label:] LOOP
	循环执行的语句
END LOOP [loop_label]

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其中，loop_label表示LOOP语句的标注名称，该参数可以省略。

举例一：

delimiter //
create procedure test_loop()
begin
		# 声明变量
		declare num int default 1;
		
		soberw:loop
				# 重新赋值
				set num = num + 1;
				if num >= 10 
						then leave soberw;
				end if;
		end loop soberw;
		
		# 查看num
		select num;
end //
delimiter ;

call test_loop();

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MySQL learning talks about process control and cursors

举例二： -- 当市场环境变好时，公司为了奖励大家，决定给大家涨工资。 -- 声明存储过程“update_salary_loop()”，声明OUT参数num，输出循环次数。 -- 存储过程中实现循环给大家涨薪，薪资涨为原来的1.1倍。 -- 直到全公司的平均薪资达到12000结束。 -- 并统计循环次数。

delimiter //
create procedure update_salary_loop(out num int)
begin
    # 声明变量
		# 记录保存平均薪资
		declare avg_sal double default 0;
		# 记录循环次数
		declare count int default 0;
		# 获取当前平均薪资
		select avg(salary) into avg_sal from employees;
		
		soberw:loop
				# 结束条件
				if avg_sal >= 12000 
								then leave soberw;
						end if;
				# 更新工资
				update employees set salary = salary * 1.1;
				# 保证当前平均薪资为最新
				select avg(salary) into avg_sal from employees;
				# 记录次数
				set count = count + 1;
		end loop soberw;
		# 返回num
		set num = count;
end //
delimiter ;

call update_salary_loop(@num);
select @num;
select avg(salary) from employees;

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2MySQL learning talks about process control and cursors

WHILE语句

WHILE语句创建一个带条件判断的循环过程。WHILE在执行语句执行时，先对指定的表达式进行判断，如果为真，就执行循环内的语句，否则退出循环。WHILE语句的基本格式如下：

[while_label:] WHILE 循环条件  DO
	循环体
END WHILE [while_label];

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while_label为WHILE语句的标注名称；如果循环条件结果为真，WHILE语句内的语句或语句群被执行，直至循环条件为假，退出循环。

举例一：WHILE语句示例，i值小于10时，将重复执行循环过程

delimiter //
create procedure test_while()
begin
    # 初始化条件
		declare i int default 1;
		#循环条件
		while i < 10 do
				# 循环体略
				#迭代条件
				set i = i + 1;
		end while;
		select i;
end//
delimiter ;

call test_while();

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2MySQL learning talks about process control and cursors

举例二： -- 市场环境不好时，公司为了渡过难关，决定暂时降低大家的薪资。 -- 声明存储过程“update_salary_while()”，声明OUT参数num，输出循环次数。 -- 存储过程中实现循环给大家降薪，薪资降为原来的90%。 -- 直到全公司的平均薪资达到5000结束。 -- 并统计循环次数。

delimiter //
create procedure update_salary_while(out num int)
begin
    # 声明变量
		# 记录保存平均薪资
		declare avg_sal double default 0;
		# 记录循环次数
		declare count int default 0;
		# 获取当前平均薪资 初始化条件
		select avg(salary) into avg_sal from employees;
		
		#循环条件
		soberw:while avg_sal > 5000 do
				# 循环体		
				# 更新工资
				update employees set salary = salary * 0.9;
				# 记录次数
				set count = count + 1;
				
				# 迭代条件
				# 保证当前平均薪资为最新
				select avg(salary) into avg_sal from employees;
				
		end while soberw;
		# 返回num
		set num = count;
end //
delimiter ;

call update_salary_while(@num);
select @num;
select avg(salary) from employees;

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2MySQL learning talks about process control and cursors

REPEAT语句

REPEAT语句创建一个带条件判断的循环过程。与WHILE循环不同的是，REPEAT 循环首先会执行一次循环，然后在 UNTIL 中进行表达式的判断，如果满足条件就退出，即 END REPEAT；如果条件不满足，则会就继续执行循环，直到满足退出条件为止。

REPEAT语句的基本格式如下：

[repeat_label:] REPEAT
　　　　循环体的语句
UNTIL 结束循环的条件表达式
END REPEAT [repeat_label]

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repeat_label为REPEAT语句的标注名称，该参数可以省略；REPEAT语句内的语句或语句群被重复，直至expr_condition为真。

举例一：

DELIMITER //

CREATE PROCEDURE test_repeat()
BEGIN	
	DECLARE i INT DEFAULT 0;
	
	REPEAT 
		SET i = i + 1;
	UNTIL i >= 10
	END REPEAT;
	
	SELECT i;
END //

DELIMITER ;

call test_repeat();

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2MySQL learning talks about process control and cursors

举例二： -- 当市场环境变好时，公司为了奖励大家，决定给大家涨工资。 -- 声明存储过程“update_salary_repeat()”，声明OUT参数num，输出循环次数。 -- 存储过程中实现循环给大家涨薪，薪资涨为原来的1.15倍。 -- 直到全公司的平均薪资达到13000结束。 -- 并统计循环次数。

delimiter //
create procedure update_salary_repeat(out num int)
begin
		# 声明变量
		# 记录保存平均薪资
		declare avg_sal double default 0;
		# 记录循环次数
		declare count int default 0;
		# 获取当前平均薪资 初始化条件
		select avg(salary) into avg_sal from employees;
		
		#循环条件
		soberw:repeat  
				# 循环体		
				# 更新工资
				update employees set salary = salary * 1.15;
				# 记录次数
				set count = count + 1;
				
				# 迭代条件
				# 保证当前平均薪资为最新
				select avg(salary) into avg_sal from employees;
				
		until avg_sal >= 13000
		end repeat soberw;
		# 返回num
		set num = count;
end //
delimiter ;

call update_salary_repeat(@num);
select @num;
select avg(salary) from employees;

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2MySQL learning talks about process control and cursors

对比三种循环结构：
1、这三种循环都可以省略名称，但如果循环中添加了循环控制语句（LEAVE或ITERATE）则必须添加名称。 2、 LOOP：一般用于实现简单的"死"循环 WHILE：先判断后执行 REPEAT：先执行后判断，无条件至少执行一次

跳转语句

跳转语句可以协助我们更好的控制循环。

LEAVE语句

LEAVE语句：可以用在循环语句内，或者以 BEGIN 和 END 包裹起来的程序体内，表示跳出循环或者跳出程序体的操作。如果你有面向过程的编程语言的使用经验，你可以把 LEAVE 理解为 break。

基本格式如下：

LEAVE 标记名

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其中，label参数表示循环的标志。LEAVE和BEGIN ... END或循环一起被使用。

举例1：创建存储过程 “leave_begin()”，声明INT类型的IN参数num。给BEGIN...END加标记名，并在BEGIN...END中使用IF语句判断num参数的值。

如果num<=0，则使用LEAVE语句退出BEGIN...END；
如果num=1，则查询“employees”表的平均薪资；
如果num=2，则查询“employees”表的最低薪资；
如果num>2，则查询“employees”表的最高薪资。

IF语句结束后查询“employees”表的总人数。

delimiter //
create procedure leave_begin(in num int)
soberw:begin
		if num <= 0 
				then leave soberw;
		elseif num = 1
				then select avg(salary) from employees;
		elseif num = 2
				then select min(salary) from employees;
		elseif num > 2
				then select max(salary) from employees;
		end if;
		select count(1) from employees;
end//
delimiter ;

call leave_begin(2);
call leave_begin(-1);

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2MySQL learning talks about process control and cursors

MySQL learning talks about process control and cursors

举例2： -- 当市场环境不好时，公司为了渡过难关，决定暂时降低大家的薪资。 -- 声明存储过程“leave_while()”，声明OUT参数num，输出循环次数， -- 存储过程中使用WHILE循环给大家降低薪资为原来薪资的90%， -- 直到全公司的平均薪资小于等于10000， -- 并统计循环次数。

delimiter//
create procedure leave_while(out num int)
begin
		declare avg_sal double;
		declare count int default 0;
		
		select avg(salary) into avg_sal from employees;
		soberw:while true do
		
				if(avg_sal <= 10000) 
						then leave soberw;
				end if;
				
				update employees set salary = salary * 0.9;
				select avg(salary) into avg_sal from employees;
				set count = count + 1;
				
		end while soberw;
		
		set num = count;
end//
delimiter;

call leave_while(@num);
select @num;
select avg(salary) from employees;

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3MySQL learning talks about process control and cursors

ITERATE语句

ITERATE语句：只能用在循环语句（LOOP、REPEAT和WHILE语句）内，表示重新开始循环，将执行顺序转到语句段开头处。如果你有面向过程的编程语言的使用经验，你可以把 ITERATE 理解为 continue，意思为“再次循环”。

语句基本格式如下：

ITERATE label

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label参数表示循环的标志。ITERATE语句必须跟在循环标志前面。

举例一：定义局部变量num，初始值为0。循环结构中执行num + 1操作。 -- 如果num < 10，则继续执行循环； -- 如果num > 15，则退出循环结构；

delimiter //
create procedure test_iterate()
begin
		declare num int default 0;
		soberw:loop
				set num = num + 1;
				if num < 10
				   then iterate soberw;
			  end if;
				if num >15
					then leave soberw;
			  end if;
		end loop soberw;
		
		select num;
end//
delimiter ;

call test_iterate();

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3MySQL learning talks about process control and cursors

游标

虽然我们也可以通过筛选条件 WHERE 和 HAVING，或者是限定返回记录的关键字 LIMIT 返回一条记录，但是，却无法在结果集中像指针一样，向前定位一条记录、向后定位一条记录，或者是随意定位到某一条记录，并对记录的数据进行处理。

这个时候，就可以用到游标。游标，提供了一种灵活的操作方式，让我们能够对结果集中的每一条记录进行定位，并对指向的记录中的数据进行操作的数据结构。游标让 SQL 这种面向集合的语言有了面向过程开发的能力。

在 SQL 中，游标是一种临时的数据库对象，可以指向存储在数据库表中的数据行指针。这里游标充当了指针的作用，我们可以通过操作游标来对数据行进行操作。

MySQL中游标可以在存储过程和函数中使用。

游标使用的步骤：

1)声明游标 DECLARE cursor_name CURSOR FOR select_statement;
2)打开游标 OPEN cursor_name
3)使用游标（从游标中获取数据） FETCH cursor_name INTO var_name [, var_name] ...
4)关闭游标 CLOSE cursor_name

注意：游标的查询结果集中的字段数，必须跟 INTO 后面的变量数一致，否则，在存储过程执行的时候，MySQL 会提示错误。
有 OPEN 就会有 CLOSE，也就是打开和关闭游标。当我们使用完游标后需要关闭掉该游标。因为游标会占用系统资源，如果不及时关闭，游标会一直保持到存储过程结束，影响系统运行的效率。而关闭游标的操作，会释放游标占用的系统资源。

举例： -- 创建存储过程“get_count_by_limit_total_salary()”， -- 声明IN参数 limit_total_salary，DOUBLE类型； -- 声明OUT参数total_count，INT类型。 -- 函数的功能可以实现累加薪资最高的几个员工的薪资值， -- 直到薪资总和达到limit_total_salary参数的值， -- 返回累加的人数给total_count。

delimiter //
create procedure get_count_by_limit_total_salary(in limit_total_salary double,out total_count int)
begin
    # 保存薪资和
		declare sum_sal double default 0;
		# 保存累加人数
		declare count int default 0;
		# 定义单个工资
		declare emp_salary double default 0;
		# 定义游标
		declare cursor_sal cursor for select salary from employees order by salary desc;
		# 打开游标
		open cursor_sal;
		# 使用游标
		while sum_sal < limit_total_salary do
		   fetch cursor_sal into emp_salary;
			set sum_sal = sum_sal + emp_salary;
			set count = count + 1;
		end while;
		#关闭游标
		close cursor_sal;
		# 给total_count赋值
		set total_count = count;
end//
delimiter ;

set @limit_total_salary = 200000;
call get_count_by_limit_total_salary(@limit_total_salary,@total_count);
select @total_count;

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3MySQL learning talks about process control and cursors

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