Mysql database foundation: process control

Process control

1、 grammar

1.1 Sequential structure

Too simple , Too lazy to write.

1.2 Branching structure

# ==========================================
# if function ：
#  grammar ：if( expression 1, expression 2, expression 3)
#  explain ： If the expression 1 establish , So execute the expression 2, Otherwise, execute the expression 3
#  If the comparison result is true, select the left , Select the right side for the false 
SELECT IF(10<5,' Big ',' Small '); #  Small 
#  for example ：
SELECT last_name,commission_pct,IF(commission_pct IS NULL,' No bonus , ha-ha ',' Bonus , Joy ')  remarks  FROM employees; 

#========================================
#  Multiple branches 
# if...else... function 
#  grammar ：
/* if  Conditions 1 then  sentence 1; elseif  Conditions 2 then  sentence 2; ... 【else  sentence n;】 end if; */
#  applications ： Apply to begin end in 

# ======================================
# case  function 

# Use 1： case Be similar to switch The effect of 
/* case  The field or expression to determine  when  Constant 1 then  The value to display 1【 Or words 1;】 when  Constant 2 then  The value to display 2【 Or words 2;】 ... else  The value to display n【 Or words n;】 end【case;】 AS  name  FROM  Table name ; */
#  for example ：
/*  Case study ：  Check the salary of employees , The requirements are as follows ：  Department number =30, The wage shown is 1.1 times   Department number =30, The wage shown is 1.2 times   Department number =40, The wage shown is 1.3 times   Other departments , The wage shown is the original wage  */
#  Realization ：
SELECT 
		salary  The original wage ,department_id,
CASE 
		department_id
WHEN 30 THEN salary*1.1
WHEN 40 THEN salary*1.2
WHEN 50 THEN salary*1.3
ELSE 
		salary
END AS  New pay 
FROM 
		employees;
#  The new salary is listed at the end of the queried table 

# Use 2： Similar to multiple if
/* case when  Conditions 1 then  The value to display 1【 Or words 1;】 when  Conditions 2 then  The value to display 2【 Or words 2;】 ... else  The value to display n【 Or words n;】 end 【case;】 */

#  for example ：
/* Case study ：  Check the salary of employees   If wages >20000, Show A Level   If wages >15000, Show B Level   If wages >10000, Show C Level   Otherwise, it will show D Level  */
#  Realization 
SELECT 
		salary  Wages ,
CASE
WHEN salary>20000 THEN 'A'
WHEN salary>15000 THEN 'B'
WHEN salary>10000 THEN 'C'
ELSE 
		'D'
END AS  Salary level 
FROM 
		employees;

It should be noted that ,case In structure , If then perhaps else Followed by ordinary values , There is no need to add a semicolon , If the sentence is followed , that then You need a semicolon after it ,end We need to add 【case;】

• Loop structure
  classification ：while、loop、repeat
  Cycle control ：
  （1）iterate, Be similar to java in continue, It means the end of this cycle , So let's go to the next loop ,iterate Followed by a circular label
  （2）leave, Be similar to java Medium break, Indicates the end of the current loop ,leave Followed by a circular label
  Be careful ： Loop structures need to be placed in begin end In the sentence （ Stored procedure or function begin end in ）

# ====================================
# while
#  grammar ：
/* 【 label :】 while  The loop condition  do  The loop body ; end while【 label 】; */

# ====================================
# repeat, Be similar to do while
#  grammar 
/* 【 label :】 repeat  The loop body ; until  Conditions for ending the cycle  end repeat 【 label 】; */

# ====================================
# loop, An unconditional loop 
#  grammar 
/* 【 label :】 loop  The loop body ; end loop【 label 】  It can be used to simulate a simple dead cycle  */

2、 exercises

# ===============================================
#  establish admin surface 
CREATE TABLE admin1(
	id INT PRIMARY KEY,
	username VARCHAR(10) NOT NULL,
	password INT NOT NULL
);

select * from admin1;

# -----------------------------------------------
#  exercises 1： Go to admin Insert in table 100 Data （ Loop structure implementation ）
#  Set the terminator to $$
delimiter $$
CREATE PROCEDURE pro_while(IN insetCount INT)
BEGIN
	#  Declare the variable to be given 
	DECLARE i INT DEFAULT 1;
	WHILE i<=insetCount DO
		INSERT INTO admin1(id,username,password) values (i,CONCAT('rose',i),666);
		SET i = i+1;
	END WHILE;
END
$$
#  Reset the end character to ;
delimiter ;

#  Execute stored procedures 
CALL pro_while(100);

#  Delete stored procedure 
DROP PROCEDURE pro_while;

# -----------------------------------------
#  exercises 2： Delete admin1 In the table id Even data 
delimiter $$
CREATE PROCEDURE pro_while_delete()
BEGIN
	DECLARE i INT DEFAULT 0;
	#  Assign the query result to @numbers User variables 
	SELECT count(*) FROM admin1 INTO @numbers;
	sign:WHILE i<@numbers DO
		SET i=i+1;
		#  If it's not even , So use iterate End this cycle , Into the next loop 
		IF MOD(i,2)!=0 THEN ITERATE sign;
		END IF;
		#  Delete id=i( even numbers ) The data of 
		DELETE FROM admin1 WHERE id=i;
	END WHILE sign;
END
$$

delimiter ;
CALL pro_while_delete();
DROP PROCEDURE pro_while_delete;

#------------------------------------------
#  exercises 3： Inserts a specified number of random strings into the specified table 
#  Title Description 
/*  Table name :stringcontent  Which field  id： Self growth  content varchar(20)  requirement ： Insert a specified number of random strings into the table  */

#  Create table stringcontent
CREATE TABLE stringcontent(
	id INT PRIMARY KEY AUTO_INCREMENT,
	content VARCHAR(20)
);

DROP TABLE IF EXISTS stringcontent;

delimiter $$
CREATE PROCEDURE test_randstr_insert(IN insertCount INT)
BEGIN
	DECLARE i INT DEFAULT 1; # Control cycle 
	DECLARE str VARCHAR(26) DEFAULT 'abcdefghijklmnopqrstuvwxyz'; # 26 A string of letters 
	DECLARE startIndex INT DEFAULT 1; #  Starting index 
	DECLARE len INT DEFAULT 1; #  The length of the intercepted character 
	WHILE i<=insertCount DO

			#  Generate a random integer , Represents the starting index of the string 1~26
			SET startIndex = FLOOR(RAND()*26+1);
			#  Generate a random integer , Used to represent the intercept length 
			SET len = FLOOR(RAND()*(26-startIndex+1)+1);

			SET i = i+1;

			#  Insert the intercepted string into the table 
			INSERT INTO stringcontent(content) values(SUBSTR(str,startIndex,len));
	
	END WHILE;
END
$$

delimiter ;
#  Calling stored procedure 
CALL test_randstr_insert(10);