Mysql stored procedure acquaintance

Stored Procedures

understanding

In some programming languages, such as pascal, there is a concept called "process" procedure, and "function" function, as in VB sub. Java, Python, PHP, there is no process, only function.

Procedure (Procedure) : encapsulates multiple statements, when invoked, perform these packages.

Function (function) : There is a return value of "process" (ps: Python function can be process, but also is a function)

Stored procedure (sql_procedure) : The sql encapsulates several pieces, a name, is the process , the process is stored in the database, i.e., the stored procedure .

Stored Procedures - Creating grammar

-- 创建
create procedure procedureName()
begin
    SQL语句1;
    SQL语句2;.....
end

-- 调用
call procedureName();

The first stored procedure

helloWorld

-- 第一个存储过程: 打印hello world
delimiter //
drop procedure if exists p1;
create procedure p1()
begin
    select "hello world!";
    select 1 + 1;
end //
delimiter ;

-- CALL 调用
call p1;
-- 查看: show procedure status;
show show procedure status;

effect

mysql> -- 第一个存储过程: 打印hello world
delimiter //
drop procedure if exists p1;
create procedure p1()
begin
    select "hello world!";
    select 1 + 1;
end //
delimiter ;
Query OK, 0 rows affected (0.16 sec)

Query OK, 0 rows affected (0.16 sec)

mysql> call p1();
+--------------+
| hello world! |
+--------------+
| hello world! |
+--------------+
1 row in set (0.06 sec)

+-------+
| 1 + 1 |
+-------+
|     2 |
+-------+
1 row in set (0.21 sec)

Query OK, 0 rows affected (0.00 sec)

The introduction of a local variable -declare

Stored procedures are programmable, meaning you can use variables, expressions, control structures to complete a variety of complex functions.

During storage, with declare variable type variable name [default Default] .

-- 变量引入
drop procedure if exists p2;
delimiter //
create procedure p2()
begin
    declare age int default 18;
    declare height int default 180;
    -- concat 拼接输出
    select concat("油哥的年龄是:", age, "身高是:", height);
end //
delimiter ;

call p2();

effect:

call p2();
Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

+-------------------------------------------------+
| concat("油哥的年龄是:", age, "身高是:", height) |
+-------------------------------------------------+
| 油哥的年龄是:18身高是:180                       |
+-------------------------------------------------+
1 row in set (0.10 sec)

Query OK, 0 rows affected (0.04 sec)

The introduction of operation

During storage, variables can be introduced into the sql statement lawful operations, such as + - * /, note that the value of that result of the operation, how to assign the variables?

set variable name: = expression variable stored in the process is a local variable.

set @ variable name: = expression user (session) variables can also be used outside of a stored procedure, similar to the "global variables."

declare variable type variable name [default value] for use in the process of local variables declared type.

= On assignment with: the difference =

:=

• Standard assignment symbols are assigned in any scenario.

=

• And update is only set and: = is the same as an assignment , the other is equal role.

drop procedure if exists p3;
delimiter //
create procedure p3()
begin
    declare age int default 18;

    select concat("现在的年龄是:", age);
    -- 变量运算,赋值
    set age := age + 10;
    select concat("10年后, 年龄变成了:", age); 
end //
delimiter ;

-- out
mysql> call p3();
+------------------------------+
| concat("现在的年龄是:", age) |
+------------------------------+
| 现在的年龄是:18              |
+------------------------------+
1 row in set (0.11 sec)

+------------------------------------+
| concat("10年后, 年龄变成了:", age) |
+------------------------------------+
| 10年后, 年龄变成了:28              |
+------------------------------------+
1 row in set (0.25 sec)

Control structure if - then - else - end if;

-- 语法
if condition then
    statement_01
else
    statement_02
end if;

drop procedure if exists p4;
delimiter //
create procedure p4()
begin
    declare age int default 18;
    
    if age >= 18 then
        select "已成年";
    else
        select "未成年";
    end if;
end //
delimiter ;

-- test
call p4();

-- out
+--------+
| 已成年 |
+--------+
| 已成年 |
+--------+
1 row in set (0.09 sec)

Query OK, 0 rows affected (0.00 sec)

Stored procedure parameter passing

In parentheses stored procedure, you can declare the parameters, the syntax is [in / out / inout] parameter name parameter type

It indicates to the procedure in which transmission parameter ; out indicates a out its transmission parameters

-- 输入矩形的 width, height 求矩形的面积
drop procedure if exists p5;
delimiter //
create procedure p5(width int, height int)
begin
    select concat("面积是:", width * height);
    if width > height then
        select "比较胖";
    elseif width < height then
        select "比较瘦";
    else
        select "方的一痞";
    end if;
    
end //
delimiter ;

call p5(12, 13);

-- out
call p5(12, 13);
Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.05 sec)

+-----------------------------------+
| concat("面积是:", width * height) |
+-----------------------------------+
| 面积是:156                        |
+-----------------------------------+
1 row in set (0.11 sec)

+--------+
| 比较瘦 |
+--------+
| 比较瘦 |
+--------+
1 row in set (0.22 sec)

Query OK, 0 rows affected (0.01 sec)

Process control while, repeat, loop

Any programming language, as long as the structure includes a control sequence, selection cycle is sufficient.

Feeling is, in fact, programming ideas are the same, but different application scenarios of language, syntax features differ only, the idea is the same.

-- while 循环 语法
WHILE search_condition DO
    statement_list
END WHILE [end_label]

-- 求 1+2+3+...100
drop procedure if exists p6;
delimiter //
create procedure p6()
begin
    declare total int default 0;
    declare num int default 0;
    -- while 循环
    while num <= 100 do
        set total := total + num;
        set num := num + 1;
    end while;
    -- 最后输出结果
    select concat("1+2+...100的值是: ", total) as 'sum';
end //
delimiter ;

call p6();

-- out
call p6();
Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.05 sec)

+------------------------+
| sum                    |
+------------------------+
| 1+2+...100的值是: 5050 |
+------------------------+
1 row in set (0.09 sec)

Improvement: seeking 1 + 2 + .... N and, where the introduction of parameters IN

-- 求 1+2+3+...N
drop procedure if exists p7;
delimiter //
-- 传入参数 in类型
create procedure p7(in n int)
begin
    declare total int default 0;
    declare num int default 0;
    -- while 循环
    while num <= n do
        set total := total + num;
        set num := num + 1;
    end while;
    -- 最后输出结果
    select concat("1+2+.. 的值是: ", total) as 'sum';
end //
delimiter ;

call p7(10000);

-- out
call p7(10000);
Query OK, 0 rows affected (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

+-------------------------+
| sum                     |
+-------------------------+
| 1+2+.. 的值是: 50005000 |
+-------------------------+
1 row in set (0.14 sec)

out -type parameters

drop procedure if exists p8;
delimiter //
create procedure p8(in n int, out total int)
begin
    -- 声明一个局部(临时)变量num来存储 1..n
    declare num int default 0;
    -- while 
    while num <= n do
        set total := total + num;
        set num := num + 1;
    end while;
    -- select concat("the sum is:", total)
end //
delimiter ;

-- 区别: 没有在 begin ..end 中声明 total变量, 而是在 OUT类型的参数中.
-- out: 传入一个变量去接收输出
call p8(100, @cj); 

mysql> select @cj;
+------+
| @cj  |
+------+
| NULL |
+------+
1 row in set (0.10 sec)

NULL specificity, resulting in no output correctly resolve: to total a default value

mysql> select null = null;
+-------------+
| null = null |
+-------------+
| NULL        |
+-------------+
1 row in set (0.09 sec)

mysql> select 1 + null;
+----------+
| 1 + null |
+----------+
| NULL     |
+----------+
1 row in set (0.05 sec)

-- 解决null的特殊性
drop procedure if exists p8;
delimiter //
create procedure p8(in n int, out total int)
begin

    -- 先声明一个局部(临时)变量num来存储 1..n
    declare num int default 0;
    -- 再给out变量一个默认值即可(顺序是先declare哦)
    set total := 0;
    -- while 
    while num <= n do
        set total := total + num;
        set num := num + 1;
    end while;
end //
delimiter ;

-- 区别: 没有在 begin ..end 中声明 total变量, 而是在 OUT类型的参数中.

-- out: 传入一个变量去接收输出的total变量值
call p8(100, @theSum);
select @theSum;

-- out

mysql> call p8(100, @theSum);
Query OK, 0 rows affected (0.00 sec)

mysql> select @theSum;
+---------+
| @theSum |
+---------+
|    5050 |
+---------+
1 row in set (0.11 sec)

Summary parameters in and out and inout

• in type, to enter a value into, passed to the procedure in the type variable (by value)
• out type, is to enter a variable into account, the value of the variable out of the receiving procedure of the type
• inout type, and the incoming values ​​into the variable reception value passed out

-- inout 类型
drop procedure if exists p9;
delimiter //
create procedure p9(inout age int)
begin
    set age := age + 20;
end //
delimiter ;

-- call 的时候, inout, 首先要定义一个"全局(会话变量)", 然后再传入
-- out
mysql> set @age := 100;
Query OK, 0 rows affected (0.00 sec)

mysql> call p9(@age);
Query OK, 0 rows affected (0.00 sec)

mysql> select @age;
+------+
| @age |
+------+
|  120 |
+------+