Knowledge points of MySQL (13)

One 、 stored procedure

meaning ： Is a set of precompiled SQL Encapsulation of statements .

Execution process ： Stored procedures are pre stored in MySQL Server , When you need to perform , The client only needs to issue a command to the server to call the stored procedure , The server side can store the pre stored series SQL Execute all statements .

benefits ：

1. Simplified operation , Improved SQL Statement reusability , Reduce the pressure on developers
2. Reduce errors in operation , Increase of efficiency
3. Reduce network traffic （ The client does not need to put all SQL The statement is sent to the server through the network ）
4. Less SQL The risk of statement exposure to the Internet , It also improves the security of data query

And views 、 Comparison of functions ：
It has the same advantages as view , Clear 、 Security , It can also reduce the amount of network transmission . But it's different from the view , Views are virtual tables , Usually do not directly operate on the underlying data table , And stored procedures are programmed SQL, You can directly manipulate the underlying data table , Compared with the collection oriented operation , It can realize some more complex data processing .

1, classification

2, Creation and call of stored procedure

# preparation 
create database dbtest15;

use dbtest15;

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

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

select * from employees;

select * from departments;


# Create stored procedure 

delimiter $

create procedure select_all_data()
begin
        select * from employees;
end $

delimiter;

# Calls to stored procedures 
call select_all_data();

  Two 、 Storage function

Create and call storage functions

# give an example 1： Create a storage function , The name is email_by_name(), Parameter definition is null , This function queries Abel Of email, And back to , The data type is string 

delimiter //

create function email_by_name()
returns varchar(25)
    # The way 1
       deterministic       #
       contains sql        # Function properties 
       reads sql data      #  
    #
begin
      return (select email from employees where last_name='Abel');

end //

delimiter;

# Calling the storage function 
select email_by_name();

select email,last_name from employees where last_name='Abel';

# give an example 2： Create a storage function , The name is email_by_id(), Parameters of the incoming emp_id, This function queries emp_id Of email, And back to , The data type is string 

# The way 2（ Declare functions ）
set global log_bin_trust_function_creators=1;

delimiter //

create function email_by_id(emp_id int)
returns varchar(25)

begin
      return (select email from employees where employee_id=emp_id);

end //

delimiter;

# call 
select email_by_id(101); # The way 1

set @emp_id :=102;
select email_by_id(@emp_id);  # The way 2

# give an example 3： Create a storage function count_by_id(), Parameters of the incoming dept_id, This function queries dept_id Number of employees in the Department , And back to , The data type is integer 

delimiter //

create function  count_by_id(dept_id int)
returns int

begin
      return (select count(*) from employees where department_id=dept_id);

end //

delimiter;
# call 
set @dept_id :=30;
select count_by_id(@dept_id);

Compare stored functions with stored procedures ：

Besides , Storage functions can be used in query statements , Stored procedures don't work , conversely , Stored procedures are more powerful , Including the ability to perform operations on tables （ For example, create a table , Delete tables, etc ） And transaction operations , These functions are not available in storage functions . 

3、 ... and 、 View of stored procedures and functions \、 Modification and deletion

1, Storage function 、 View of stored procedure

1） Use show create Statement to view the creation information of stored procedures and functions

show create procedure Stored procedure name ：

show create function Store function name ;

2） Use show status Statement to view the status information of stored procedures and functions

show procedure status like ' Stored procedure name ';

show function status like ' Store function name ';

 3） from information_schema.routines View the stored procedure and function information in the table

select * from information_schema.routines where routine_name=' Store name ';

2,  Storage function 、 Changes to stored procedures

Be careful ： Modify stored procedures or functions , It does not affect the function of stored procedures or functions , Just modify the relevant features .

alter procedure stored procedure

SQL security invoker

comment ' Query maximum wage ';

3, Storage function 、 Deletion of stored procedure

drop function if exists Store function name ;

drop procedure if exists Stored procedure name ; 

  stored procedure ：

advantage ：

1. Stored procedures can be compiled and used more than once
2. It can reduce the development workload
3. The security of stored procedure is strong
4. It can reduce the amount of network transmission
5. Good encapsulation

shortcoming ：

1. Poor portability
2. Debugging difficulty
3. Version management of stored procedures is difficult
4. It is not suitable for high concurrency scenarios

Four 、LOOP、WHITE、REPEAT Three circulation structures

1,LOOP loop

LOOP Loop statements are used to repeat certain statements ,LOOP The statements in the loop are repeated until the loop is exited （ Use LEAVE Clause ）, Jump out of the loop process .

# give an example 1

delimiter //
create procedure test_loop()
begin
      # Declare local variables 
      declare num int default 1;
      
      loop_label:loop
           # Reassign 
           set num=num+1;
           if num>=10 then leave loop_label;
           end if;
      end loop loop_label;
      
      # see num
      select num;

end //

delimiter;

# call 

call test_loop();

# give an example 2： Declaring stored procedures “update_salary_loop()", Statement out Parameters num, Number of output cycles , Loop in the stored procedure to raise everyone's salary , Original 1.1 times , Until the average salary of the whole company reaches 12000 end , And count the number of cycles 

delimiter //

create procedure update_salary_loop(out num int)
begin
      # Declare variables 
      declare avg_sal double;
      declare loop_count int default 0; # Record times 
      
      # Get the average salary of employees 
      select avg(salary) into avg_sal from employees;

      loop_lab:loop
              # End cycle condition 
              if avg_sal>=12000 then leave loop_lab;
              end if;
  
              # If lower 12000, Update the salary of personnel 
              update employees set salary =salary * 1.1;

              # to update avg_sal The value of the variable 
              select avg(salary) into avg_sal from employees;
              # Record the number of cycles 
              set loop_count=loop_count+1;

      end loop loop_lab;

      # to num assignment 
      set num=loop_count;

end //

delimiter;

# call 
call update_salary_loop(@num);

select @num;

2,while loop

# give an example 1：
delimiter //

create procedure test_while()

begin
      # Initialization conditions 
      declare num int default 1;
      while num<=10 do

       # Iteration conditions 
        set num=num+1;
      end while;

     # Inquire about 
     select num;

end //

delimiter;

# call 
call test_while();

3,repeat loop

# give an example 
delimiter //

create procedure test_repeat()
begin
      # Declare variables 
      declare num int default 1;

      repeat
             set num=num+1;
       until num>=10
       end repeat;

       # see 
       select num;

end //

delimiter ;

# call 
call test_repeat();

Compare the three cycles ：
LOOP： Generally used to implement simple ” die “ loop .

WHILE： Judge before you execute .

REPEAT： Execute before judge , Unconditionally execute at least once .

5、 ... and 、 The cursor （ Or the cursor ）

Cursor let SQL This set oriented language has the ability of process oriented development

Steps for using cursors ：

1. declare cursor
2. Open cursor
3. Use cursors （ Get data from cursor ）
4. Close cursor

 give an example 1： Create stored procedure “get_count_by_limit_total_salary()", Statement in Parameters  limit_total_salary,double type ： Statement out Parameters total_count,int type . The function can accumulate the salary values of several employees with the highest salary , Until the total salary reaches limit_total_salary The value of the parameter , Return the accumulated number of people to total_count.

delimiter //

create procedure get_count_by_limit_total_salary(in limit_total_salary double,out total_count int)

begin
       # Declare variables 
       declare sum_sal double default 0.0;  # Record the total accumulated salary 
       declare emp_sal double;              # Record the salary of each employee 
       declare emp_count int default 0;     # Record the cumulative number of people 


       # declare cursor 
       declare emp_cursor cursor for select salary from employees order by salary desc;
       
       # Open cursor 
       open emp_cursor;

       
       repeat 

           # Use cursors 
           fetch emp_cursor into emp_sal;
           set sum_sal= sum_sal+ emp_sal;
           set emp_count=emp_count +1;
           until sum_sal >= limit_total_salary

       end repeat;

       set total_count =emp_count;
 
       # Close cursor 
       close emp_cursor;

end //

delimiter;

# call 
call get_count_by_limit_total_salary(200000,@total_count);

select @total_count;

Summary ： The cursor is MySQL An important function of , To read the data in the result set one by one , Provides the perfect solution , Compared with implementing the same function at the application level , Cursors can be used in stored programs , Efficient , The program is also more concise , But it needs to be closed after use , This can improve the overall efficiency of the system .