Knowledge points in T-SQL

local variable

• Local variables must be marked with @ The prefix , Such as @age
• The use of local variables is also declared first , To assign a value

DECLARE @ Variable name   data type 
DECLARE @Name varcher(8)
DECLARE @seat int

assignment

The syntax of assignment is as follows ：

SET @ Variable name  =  value  -- similar UPDATE  It is written in the same way as other assignment statements , Assign the value after the equal sign to the variable before the equal sign .
 or  
SELECT @ Variable name  =  value 

In operations involving any local variable , Must be @ Put the character before the variable name .

Example ：

SET @name = ' Zhang San '
SELECT @name = studentName From Student 
WHERE studentNo = '10011';

SET Statements can only be assigned directly , Not to SELECT Statement to directly assign the query result to a variable .
SELECT Statements can be assigned directly .

• SELECT Statement can assign values to multiple variables at the same time , and SET Statement can assign values to only one variable at a time .
• When an expression returns multiple values ,SET Statements will report errors , and SELECT The last value is assigned to the variable .
• When the expression does not return a value ,SET The statement will be assigned to NULL,SELECT Statement will make the variable

Global variables

• Global variables must be marked with @@ The prefix , Such as @@error
• Global variables are defined and maintained by the system , Can only read , Do not modify , And the user cannot declare the definition

Even if I write @@ It is not a global variable
His writing @@ Variable names can also be assigned , But the system will think @ @ Variable name

Examples are as follows ：

declare @@email varchar(10)
set @@email ='abc'
select @@email
-- The compiler can tell the difference ,@@email System identification as @ @email  So it is not a global variable but a local variable 

This is the basic global variable ：

@@ERROR： the last one T-SQL Wrong error number
@@IDENTITY ： Last inserted identification value
@@SERVERNAME： Name of the local server
@@VERSION：SQL Server Version information for

Data type conversion

To provide solutions to data type incompatibilities , The database provides a link for data type conversion . There are several ways .

cast The way ：

CAST（ expression AS data type )

Examples are as follows ：

select 'student age is' + 10 -- Report errors 
select 'student age is' + CAST(10 as varchar )-- Find out the data  student age is 10

convert The way ：

CONVERT( data type , expression , style )// The data type is the converted data type

Examples are as follows ：

select 'student age is '+10; -- Report errors 
select 'student age is '+ CONVERT(varchar,10,133) -- Find out the data  student age is 10

But in the database "+" No. focuses more on computation than string splicing , If it's this form ：

select '10'+10 -- The query result is 20
select CONVERT(int,10)+10 --  The query result is  20, Be careful int  There are no extra styles in the type , So the style can be omitted .

Because of "+" More emphasis on calculation , It will not report an error but directly calculate the result .

Following example ：

select CONVERT(varchar,RegisterTime) from users -- Convert date format to string format output 
select CONVERT(varchar,RegisterTime,110) from users -- Change the format of the detected string to 110 style 
select CONVERT(varchar,RegisterTime,2222) from users -- Style values have ranges, not just any one .

It shows that the style value has a range . And this style is mainly for the display of dates .

cast and convert The difference between ：

cast There is no way to select the converted style .
and convert Prefer date format .

Branching structure

IF-ELSE sentence

take C# in if-else Replace the braces in the statement with BEGIN - END Sentence block
· ELSE It's optional
· If there are multiple statements , That's what we need BEGIN-END Sentence block .
Examples are as follows ：

declare @num int 
--set @num = 60
set @num = 70
if(@num > 60)

begin
-- If there is only one statement , You don't have to write BEGIN-END  Sentence block 
-- and C#  equally , Will automatically read immediately following if The statement after the statement 
print(@num)
print('OK')
select * from Users where UserStateId < @num
end
else
	print('error')
--else
--print('error')

IF -ELSEIF sentence
Examples are as follows ：

select * from Users
declare @num int 
--set @num = 60
set @num = 70
IF(@num > 60)

BEGIN
		print ('a')
-- If there is only one statement , You don't have to write BEGIN-END  Sentence block 
-- and C#  equally , Will automatically read immediately following if The statement after the statement 
--print(@num)
--print('OK')
--select * from Users where UserStateId < @num
END
	else if( @num = 60)
BEGIN
		print('b')
END
	ELSE
BEGIN
		print('c')
END

CASE - END sentence

The grammar is as follows

CASE
WHEN Conditions 1 THEN result 1
WHEN Conditions 2 THEN result 2
…
ELSE Other results
END

When not satisfied WHEN All the conditions of , use ELSE
ELSE： It means as if all WHEN The conditions are not TRUE If omitted, the result returned by ELSE And WHEN The conditions are FALSE when ,CASE Statement returns NULL
THEN It is not allowed to write other logical operations after .
CASE-END In the sentence , As long as one condition is met , The following conditions will not continue to run .
THEN Followed by a result , Suppose you follow a query statement , Returns a result set of multiple results , You're going to report a mistake . If the following query statement is a result , Then it can run successfully .
Examples are as follows ：

select * , case 
				when UserRoleId = 1 then 'A'
				when UserRoleId = 2 then 'B'
				when UserRoleId = 3 then 'C'
				else 'D'
			end as 'level'



from Users

Loop statement

While sentence

grammar ：

WHILE （ Conditions ）
BEGIN
Sentence one
Sentence two
…
BREAK
END

Examples are as follows ：

declare @sum int = 0
declare @i int = 1
while ( @i <=100)
begin
set @sum = @sum +@i
set @i= @i+1

   continue
--return
--break

end
print @sum

Batch instructions GO

The batch ：
It contains one or more SQL Groups of statements , Send one-time from the application to SQL Server perform
SQL Server To batch SQL Statement is compiled into an execution unit , This unit is called an execution plan .
Using batch processing can improve the execution effect of code , Usually, the code of related business will be stored in the same batch processing statement .
GO Must be a single line .
GO Is not a T-SQL command , Only commands recognized by the editing tool .
All statements start at the beginning of the script or the previous GO Statement starts compiling , Know the next one GO End of statement or script , Compile this code into an execution plan .