MySQL function variable stored procedure

mysql Variable

mysql Variable

1. System variables
   Global variables ： For all conversations （ Connect ） It works , But not across reboots
   Session variables ： For the current session （ Connect ） It works , Scope is the same as session variable
2. User defined variables
   User variables ： For the current session （ Connect ） It works , Scope is the same as session variable
   local variable ： For the current session （ Connect ） It works , Scope is the same as session variable

System variables

explain ： Variables are defined by the system , Not user defined , At the server level
Be careful ： Global variables need to add global keyword , Session variables need to add session keyword , If you don't write , Default session level
Use steps ：
1、 See all system variables

show global|【session】variables;

2、 Look at some of the system variables that meet the criteria

show global|【session】 variables like '%char%';

3、 View the value of the specified system variable

select @@global|【session】 System variable name ;

4、 Assign a value to a system variable
Mode one ：

set global|【session】 System variable name = value ;

Mode two ：

set @@global|【session】 System variable name = value ;       

Global variables

#① See all global variables 
SHOW GLOBAL VARIABLES;
#② Look at some of the system variables that meet the criteria 
SHOW GLOBAL VARIABLES LIKE '%char%';
#③ View the value of the specified system variable 
SELECT @@global.autocommit;
#④ Assign a value to a system variable 
SET @@global.autocommit=0;
SET GLOBAL autocommit=0;

Session variables

#2》 Session variables 
/*
 Scope ： For the current session （ Connect ） It works 
*/
#① View all session variables 
SHOW SESSION VARIABLES;
#② Look at some session variables that meet the criteria 
SHOW SESSION VARIABLES LIKE '%char%';
#③ View the value of the specified session variable 
SELECT @@autocommit;
SELECT @@session.tx_isolation;
#④ Assign a value to a session variable 
SET @@session.tx_isolation='read-uncommitted';
SET SESSION tx_isolation='read-committed';

User variables

explain ： Variables are user-defined , Not the system
Use steps ：
1、 Statement
2、 assignment
3、 Use （ see 、 Compare 、 Operations, etc ）
Scope ： For the current session （ Connect ） It works , Scope is the same as session variable

 User variables 
/*
 Scope ： For the current session （ Connect ） It works , Scope is the same as session variable 
*/
# Assignment operator ：= or :=
#① Declare and initialize 
SET @ Variable name = value ;
SET @ Variable name := value ;
SELECT @ Variable name := value ;

#② assignment （ Update the value of the variable ）
# Mode one ：
	SET @ Variable name = value ;
	SET @ Variable name := value ;
	SELECT @ Variable name := value ;
# Mode two ：
	SELECT  Field  INTO @ Variable name 
	FROM  surface ;
#③ Use （ Look at the value of the variable ）
SELECT @ Variable name ;

local variable

#2》 local variable 
/*
 Scope ： Just defining it begin end It works in blocks 
 Apply to  begin end The first sentence in 
*/

#① Statement 
DECLARE  Variable name   type ;
DECLARE  Variable name   type  【DEFAULT  value 】;


#② assignment （ Update the value of the variable ）

# Mode one ：
	SET  Local variable name = value ;
	SET  Local variable name := value ;
	SELECT  Local variable name := value ;
# Mode two ：
	SELECT  Field  INTO  Have variable names 
	FROM  surface ;
#③ Use （ Look at the value of the variable ）
SELECT  Local variable name ;

Example declaration print sum of two variables

# User variables 
SET @m=1;
SET @n=1;
SET @[email protected][email protected];
SELECT @sum;


# local variable 


DECLARE m INT DEFAULT 1;
DECLARE n INT DEFAULT 1;
DECLARE SUM INT;
SET SUM=m+n;
SELECT SUM;

stored procedure

meaning ： A set of precompiled SQL Collection of statements , Understand batch statements
1、 Improve code reuse
2、 Simplified operation
3、 Reduce the number of compilations and connections to the database server , Improved efficiency

Create Syntax

/*
1、 The parameter list consists of three parts 
 Parameter mode    Parameter name    Parameter type 
 give an example ：
in stuname varchar(20)

 Parameter mode ：
in： This parameter can be used as input , That is, the parameter requires the caller to pass in a value 
out： This parameter can be used as output , That is, the parameter can be used as the return value 
inout： This parameter can be used as input or output , That is to say, the parameter needs to pass in the value , You can return a value 

2、 If the stored procedure body has only one sentence ,begin end It can be omitted 
 Each of the stored procedure bodies sql A semicolon is required at the end of a statement .
 The end of the stored procedure can use  delimiter  To reset 
 grammar ：
delimiter  End mark 
 Case study ：
delimiter $
*/
CREATE PROCEDURE  Stored procedure name ( parameter list )
BEGIN

	 Stored procedure body （ A combination of SQL sentence ）
END

Call syntax

CALL  Stored procedure name ( Argument list );

Use cases
1, Empty parameter list

SELECT * FROM admin;

CREATE PROCEDURE myp1()
BEGIN
	INSERT INTO admin(username,`password`) 
	VALUES('john1','0000'),('lily','0000'),('rose','0000'),('jack','0000'),('tom','0000');
END 

2, Create a in Stored procedure of mode parameters

CREATE PROCEDURE myp2(IN beautyName VARCHAR(20))
BEGIN
	SELECT bo.*
	FROM boys bo
	RIGHT JOIN beauty b ON bo.id = b.boyfriend_id
	WHERE b.name=beautyName;

END 

# call 
CALL myp2(' Small zhao ')

3. establish out Stored procedure of mode parameters

CREATE PROCEDURE myp6(IN beautyName VARCHAR(20),OUT boyName VARCHAR(20))
BEGIN
	SELECT bo.boyname INTO boyname
	FROM boys bo
	RIGHT JOIN
	beauty b ON b.boyfriend_id = bo.id
	WHERE b.name=beautyName ;
	
	SET @boyname
END 
CALL myp6(' Small zhao ', @boyname)
SELECT @boyname

4. Create a inout Stored procedure of mode parameters
Pass in a and b Two values , Final a and b All double and return

CREATE PROCEDURE myp8(INOUT a INT ,INOUT b INT)
BEGIN
	SET a=a*2;
	SET b=b*2;
END 

# call 
SET @m=10
SET @n=20
CALL myp8(@m,@n)
SELECT @m,@n

Delete stored procedure
grammar ：drop procedure Stored procedure name

DROP PROCEDURE p1

View stored procedures
grammar ：SHOW CREATE PROCEDURE Stored procedure name

SHOW CREATE PROCEDURE  myp2;

function

meaning ： A set of precompiled SQL Collection of statements , Understand batch statements
1、 Improve code reuse
2、 Simplified operation
3、 Reduce the number of compilations and connections to the database server , Improved efficiency

difference ：
stored procedure ： There can be 0 Come back , There can also be multiple returns , Suitable for bulk insertion 、 Batch update
function ： Yes and no 1 Come back , It is suitable to return a result after processing data

Create Syntax

CREATE FUNCTION  Function name ( parameter list ) RETURNS  Return type 
BEGIN
	 The body of the function 
END

Be careful ：
1. parameter list It has two parts ：
Parameter name Parameter type

2. The body of the function ： There will be return sentence , If there's no mistake
If return The statement is not placed at the end of the function body and no error is reported , But not recommended

return value ;
3. There is only one sentence in the function body , You can omit begin end
4. Use delimiter Statement set end tag

Call syntax

SELECT  Function name ( parameter list )

Use cases
1. Empty parameter list

# Case study ： Number of employees returned to the company 
CREATE FUNCTION myf1() RETURNS INT
BEGIN

	DECLARE c INT DEFAULT 0;# Defining local variables 
	SELECT COUNT(*) INTO c# assignment 
	FROM employees;
	RETURN c;
	
END 

SELECT myf1()$

There are parameters and returns

#2. You can go back to 
# Case study 1： According to the name of the employee , Return its wages 
CREATE FUNCTION myf2(empName VARCHAR(20)) RETURNS DOUBLE
BEGIN
	SET @sal=0;# Define user variables  
	SELECT salary INTO @sal   # assignment 
	FROM employees
	WHERE last_name = empName;
	RETURN @sal;
END

# Case study 2： According to the Department name , Return the average wage of the Department 

CREATE FUNCTION myf3(deptName VARCHAR(20)) RETURNS DOUBLE
BEGIN
	DECLARE sal DOUBLE ;
	SELECT AVG(salary) INTO sal
	FROM employees e
	JOIN departments d ON e.department_id = d.department_id
	WHERE d.department_name=deptName;
	RETURN sal;
END

View functions
grammar ：SHOW CREATE FUNCTION Function name ;

SHOW CREATE FUNCTION myf3;

Delete function
grammar ：
DROP FUNCTION Function name ;

DROP FUNCTION myf3;

Process control structure

Branching structure
if function
grammar ：if( Conditions , value 1, value 2)
function ： Implement two branches
Apply to begin end In or out

.case structure
grammar ：
situation 1： Be similar to switch
case Variable or expression
when value 1 then sentence 1;
when value 2 then sentence 2;
…
else sentence n;
end

situation 2：
case
when Conditions 1 then sentence 1;
when Conditions 2 then sentence 2;
…
else sentence n;
end

Apply to begin end In or out

if structure

grammar ：
if Conditions 1 then sentence 1;
elseif Conditions 2 then sentence 2;
…
else sentence n;
end if;
function ： Similar to multiple if

It can only be applied to begin end in

The application case
Case study 1： Create a function , Achieve the results , If the result >90, return A, If the result >80, return B, If the result >60, return C, Otherwise return to D

CREATE FUNCTION test_if(score FLOAT) RETURNS CHAR
BEGIN
	DECLARE ch CHAR DEFAULT 'A';
	IF score>90 THEN SET ch='A';
	ELSEIF score>80 THEN SET ch='B';
	ELSEIF score>60 THEN SET ch='C';
	ELSE SET ch='D';
	END IF;
	RETURN ch;
	
	
END 

SELECT test_if(87)

Loop structure
while
grammar ：【 label :】while The loop condition do
The loop body ;
end while【 label 】;
The application case
Circular insert

DROP PROCEDURE pro_while1
CREATE PROCEDURE pro_while1(IN insertCount INT)
BEGIN
	DECLARE i INT DEFAULT 1;
	WHILE i<=insertCount DO
		INSERT INTO admin(username,`password`) VALUES(CONCAT('Rose',i),'666');
		SET i=i+1;
	END WHILE;
END