Using T-SQL data type conversion variables

Use variables

Local variables

1. @ As a prefix must be marked as @age

2. Use local variables be declared (using the keyword declare), then the assignment

   DECLARE @age INT   --声明局部变量@age
   --SET @age=20      --赋值局部变量
   SELECT @age=20
   SELECT @age        --查看局部变量的值
   

   Use SELECT assignment to ensure that screening is only one record

Global Variables

1. Tag as a prefix must @@, @@ version as
2. Can only be read by the system defined and maintained, you can not modify the global variable values

Use variables

Question: write T-SQL query Xiaoming students and their adjacent school number

DECLARE @StuId INT --声明学号变量，这个变量记录小明的学号
SELECT @StuId=ID FROM Student WHERE Name='小明'--根据学生姓名查询出改学生的学号
SELECT * FROM Student WHERE ID=(@StuId+1) OR ID=(@StuId-1)--根据这个学号查询相邻的学员信息

The difference between SET and SELECT

scenes to be used	SET	SELECT
At the same time multiple variable assignment	not support	stand by
When the expression returns multiple values	Error	The last value returned to a variable
When the expression does not return value	Variable is assigned a NULL value	Variable holds its value

DECLARE @age INT,@name VARCHAR(20)
--SET @age=20,@name='张三'--不允许这样做
SELECT @age=20,@name='张三'
SELECT @name
--SET @name=(SELECT Name FROM Student)
SELECT @name=Name FROM Student
--SET @name=(SELECT NAME FROM Student WHERE ID=0)
SELECT @name=NAME FROM Student WHERE ID=0
SELECT @name

The use of global variables

variable	meaning
@@ERROR	The last T-SQL error error number
@@IDENTITY	Last inserted identity value
@@LANGUAGE	The name of the current language used
@@MAX_CONNECTIONS	The maximum number of simultaneous connections that can be created
@@ROWCOUNT	The number of rows affected by a SQL statement on the subject
@@SERVERNAME	The name of the local server
@@TRANSCOUNT	Number of transactions currently open connections
@@VERSION	SQL Server version information of

Data type conversion

Data type conversion

1. Use the Convert function to achieve cast

   CONVERT(数据类型,表达式,样式)--第三个参数可以省略。它一般用于日期类型数据转换为字符类型，
   

   PRINT '平均年龄是：'+CONVERT(VARCHAR(20),@avgAge)
   

2. Using the function to convert CAST

   CASR(表达式 AS 数据类型)
   

   PRINT '平均年龄是：'+CAST(@avgAge AS VARCHAR(20))