Definition of variables and assignment of variables in MySQL

explain ： Now the tutorials and books on defining variables are basically on stored procedures , But variables can only act on begin…end In block , However, the definition and use of common variables are less , To solve these problems, we can only find explanations in the official documents .

Preface

MySQL Stored procedure , There are two ways to define variables ： 1、 Use set or select Direct assignment , The variable name is @ start for example ：

set @var=1; 

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You can declare... Anywhere in a conversation , The scope is the entire session , Called user variable .

2、 With declare Keyword declared variables , Can only be used in stored procedures , Called a stored procedure variable , for example ：

declare var1 int default 0; 

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Mainly used in the storage process , Or send parameters to storage .

The difference between the two ：

When calling a stored procedure , With declare Declared variables are initialized to null. And the conversation variable （ namely @ Variable at the beginning ） Will not be reinitialized , In a conversation , Just initialize once , After that, the results of the last calculation are all within the session , It's equivalent to the global variable in this session .

Main content

• local variable
• User variables
• Session variables
• Global variables

Session variables and global variables are called system variables .

One 、 local variable , Only in the present begin/end Valid in code block Local variables are generally used in sql In the block , For example, stored procedure begin/end. Its scope is limited to the statement block , After execution of the statement block , Local variables disappear .declare Statement is used specifically to define a local variable , have access to default To describe the default values .set Statement is to set different types of variables , Including session variables and global variables . Local variables define the syntax form

declare var_name [, var_name]... data_type [ DEFAULT value ];

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For example, in begin/end Add the following statement to the statement block , Accept the function passed in a/b Variables are then added , adopt set The statement is assigned to c Variable .

set Sentence grammatical form set var_name=expr [, var_name=expr]…; set Statement can be used for the assignment of local variables , It can also be used for declaration and assignment of user variables .

declare c int default 0;
set c=a+b;
select c as C;

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Or use select …. into… Form assignment

select into  Sentence pattern ：select col_name[,...] into var_name[,...] table_expr [where...];

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Example ：

declare v_employee_name varchar(100);
declare v_employee_salary decimal(8,4);

select employee_name, employee_salary
into v_employee_name, v_employee_salary
from employees
where employee_id=1;

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Two 、 User variables , The user variables are valid throughout the process of linking the client to the database instance .

MySQL The user variable in does not need to be stated in advance , Use... Directly when using “@ Variable name ” Just use it .

First usage ：set @num=1; or set @num:=1; // Use here set Statement to create and initialize variables , Use it directly @num Variable

Second usage ：select @num:=1; or select @num:= Field name from Table name where ……,

select Statement is used to output user variables , such as select @ Variable name , Data used to output data sources that are not tables .

Notice the two assignment symbols above , Use set Can be used when “=” or “:=”, But use select Must use “:= assignment ”

User variables are connected to the database of , Variables declared in the connection , After the user variable is created in the stored procedure, it will continue until the database instance is disconnected , Variables will disappear .

Variables declared in this connection cannot be used in another connection .

The variable name of the user variable takes the form @varname In the form of .

The name must be @ start .

You need to use... When declaring variables set sentence , For example, the following statement declares a statement named @a The variable of .

set @a = 1;

Declare a @a The variable of , And assign it to 1,MySQL The variables in it are not strictly limited to data types , Its data type varies according to the value you assign to it .（SQL SERVER Use in declare Statement declaration variables , And strictly limit the data type .）

We can also use select Statement to assign a value to a variable .

such as ：

set @name = '';
select @name:=password from user limit 0,1;
# Get a record from the data table password The value of the field is given to @name Variable . Output to query result set after execution .

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（ Notice that there is a colon in front of the equals sign , hinder limit 0,1 Is used to limit the return of results , It can be 0 or 1 individual . amount to SQL SERVER Inside top 1）

If you write directly ：select @name:=password from user;

If the query returns multiple values , that @name The value of the variable is the last recorded password Value of field .

User variables can act on the current entire connection , But when the current connection is broken , All user variables defined by it will disappear .

User variables are used as follows （ We don't need to use declare Keywords define user variables , It can be used directly ） Definition , The variable name must be @ Start ：

# Definition 
select @ Variable name    perhaps  select @ Variable name :=  Field name  from  Table name  where  Filter statements ;
set @ Variable name ;
# assignment  @num Variable name ,value Value 
set @num=value; or select @num:=value;

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There are two ways to assign values to user variables , One is direct use ”=” Number , The other is to use ”:=” Number . The difference lies in the use of set When the command assigns a value to a user variable , Both methods can be used ; When using select Statement when assigning a value to a user variable , Only use ”:=” The way , Because in select In the sentence ,”=” Number declare Statement is used specifically to define a local variable .set Statement is to set different types of variables , Including session variables and global variables .

for example ：

begin
#Routine body goes here...
#select c as c;
declare c int default 0;
set @var1=143;  # Define a user variable , And initialize to 143
set @var2=34;
set c=a+b;
set @d=c;
select @sum:=(@[email protected]) as sum, @dif:=(@[email protected]) as dif, @d as C;# Use user variables [email protected] Represents the variable name 

set c=100;
select c as CA;
end

# Execute the following statement segment in the query 
call `order`(12,13);  # Execute the stored procedure defined above 
select @var1;  # Look at the user variables defined after the execution of the stored procedure , Whether it can also output , The result is that user variables can be output @var1,@var2 two-variable .
select @var2;

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The execution of the order After the stored procedure , New in stored procedure var1,var2 User variables can still be used select Statement output , But the local variables defined in the stored procedure c Can't identify .

System variables ：

System variables are divided into global variables and session variables .

The global variable is in MySQL At startup, the server automatically initializes them to the default value , These default values can be changed by my.ini This file to change .

Session variables every time a new connection is established , from MySQL To initialize the .MySQL Copies the values of all current global variables . As a conversation variable .

（ in other words , If after the session is established , The values of session variables and global variables have not been changed manually , So the values of all these variables are the same .）

The difference between a global variable and a session variable is , Changes to global variables affect the entire server , But changes to session variables , It only affects the current conversation （ That is, the current database connection ）.

We can use

show session variables;

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Statement to output all session variables （ I could just write it as show variables, If you don't specify whether to output global variables or session variables , Output session variables by default .） If you want to output all global variables ：

show global variables

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The values of some system variables can be changed dynamically using statements , But some system variables are read-only .

For those system variables that can be changed , We can use set Statement changes .

There are two system variables before the variable name @;

If you want to change Session variables Value , Using statements ：

set session varname = value;
 perhaps 
set @@session.varname = value;

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such as ：

mysql> set session sort_buffer_size = 40000;
Query OK, 0 rows affected(0.00 sec)
 use select @@sort_buffer_size; Output to see what the changed value is .
 If you want to change the value of a global variable , take session Change to global：
set global sort_buffer_size = 40000;
set @@global.sort_buffer_size = 40000;

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But to change the value of a global variable , Need to own super jurisdiction .

（ Be careful ,root Just a built-in account , Not a kind of authority , This account has MySQL All permissions in the database . Any account as long as it has the name super The authority of , You can change the value of the global variable , As long as any user has file Permission can be called load_file perhaps into outfile,into dumpfile,load data infile equally .）

utilize select We can Query the value of a single session variable or a global variable ：

select @@session.sort_buffer_size
select @@global.sort_buffer_size
select @@global.tmpdir

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Whatever is mentioned above session, Both can be used. local This keyword replaces .

such as ：

select @@local.sort_buffer_size
local yes session A synonym for .

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No matter when setting system variable or querying system variable value , As long as you don't specify whether it is a global variable or a session variable . All as session variables .

such as ：

set @@sort_buffer_size = 50000; 
select @@sort_buffer_size; 

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None of the above is designated as blobal still session, So all as session Handle .

3、 ... and 、 Session variables

The server maintains a series of session variables for each connected client . When the client connects to the database instance , Use the current value of the corresponding global variable to initialize the client's session variable . Setting session variables does not require special permissions , But the client can only change its own session variables , The session variables of other clients cannot be changed . Session variables have the same scope as user variables , Current connection only . When the current connection is disconnected , All session variables it sets are invalid .

There are three ways to change the value of a session variable ：

set session var_name = value;
set @@session.var_name = value;
set var_name = value;  # default session The default keyword is session

 View all session variables 
show session variables;

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There are three ways to view a session variable ：

select @@var_name;
select @@session.var_name;
show session variables like "%var%";

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Whatever is mentioned above session, Both can be used. local This keyword replaces .

such as ：

select @@local.sort_buffer_size 
local yes session A synonym for .

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Four 、 Global variables

Global variables affect the overall operation of the server . When the server starts , It initializes all global variables to default values . These defaults can be changed in the options file or in the options specified on the command line . To change global variables , Must possess super jurisdiction . The global variable acts on server The entire life cycle , But not across reboots . That is to say, all global variables set after restart are invalid . For global variables to continue to take effect after restart , The corresponding configuration file needs to be changed .

To set a global variable , There are two ways ：

set global var_name = value; // Be careful ： Here global Don't omit . According to the manual ,set Command to set variables if you do not specify GLOBAL、SESSION perhaps LOCAL, By default SESSION
set @@global.var_name = value; // ditto 

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View all global variables

show global variables; 

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To see a global variable , There are two ways ：

select @@global.var_name;
show global variables like “%var%”;

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