Create table variable with exact same structure of

问题:

In T-SQL, I can create a table variable using syntax like

DECLARE @table AS TABLE (id INT, col VARCHAR(20))

For now, if I want to create an exact copy of a real table in the database, I do something like this

SELECT * 
FROM INFOMATION_SCHEMA.COLUMNS 
WHERE TABLE_NAME = 'MY_TABLE_NAME'

to check the column datatype and also max length, and start to create the @table variable, naming the variable, datatype and max_length one by one which is not very effective. May I know if there is any simpler way to do it like

DECLARE @table AS TABLE = SOME_REAL_TABLE_IN_DATABASE

Furthermore, is there any way to retrieve the column name, data type and max length of the column and use it directly in the declaration like

DECLARE @table AS TABLE (@col1_specs)

Thank you in advance.

EDIT: Thanks for the answers and comments, we can do that for @table_variable but only in dynamic SQL and it is not good for maintainability. However, we can do that using #temp_table.

Based on the answer by Ezlo, we can do something like this :

SELECT TABLE.* INTO #TEMP_TABLE FROM TABLE

For more information, please refer to this answer.

Difference between temp table and table variable (stackoverflow)

Difference between temp table and table variable (dba.stackexchange)

回答1:

Object names and data types (tables, columns, etc.) can't be parameterized (can't come from variables). This means you can't do the following (which would be required to copy a table structure, for example):

DECLARE @TableName VARCHAR(50) = 'Employees'

SELECT
    T.*
FROM
    @TableName AS T

The only workaround is to use dynamic SQL:

DECLARE @TableName VARCHAR(50) = 'Employees'

DECLARE @DynamicSQL VARCHAR(MAX) = '
    SELECT
        T.*
    FROM
        ' + QUOTENAME(@TableName) + ' AS T '

EXEC (@DynamicSQL)

However, variables (scalar and table variables) declared outside the dynamic SQL won't be accessible inside as they lose scope:

DECLARE @VariableOutside INT = 10

DECLARE @DynamicSQL VARCHAR(MAX) = 'SELECT @VariableOutside AS ValueOfVariable'

EXEC (@DynamicSQL)

Msg 137, Level 15, State 2, Line 1
Must declare the scalar variable "@VariableOutside".

This means that you will have to declare your variable inside the dynamic SQL:

DECLARE @DynamicSQL VARCHAR(MAX) = 'DECLARE @VariableOutside INT = 10
                                    SELECT @VariableOutside AS ValueOfVariable'

EXEC (@DynamicSQL)

Result:

ValueOfVariable
10

Which brings me to my conclusion: if you want to dynamically create a copy of an existing table as a table variable, all the access of your table variable will have to be inside a dynamic SQL script, which is a huge pain and has some cons (harder to maintain and read, more prone to error, etc.).

A common approach is to work with temporary tables instead. Doing a SELECT * INTO to create them will inherit the table's data types. You can add an always false WHERE condition (like WHERE 1 = 0) if you don't want the actual rows to be inserted.

IF OBJECT_ID('tempdb..#Copy') IS NOT NULL
    DROP TABLE #Copy

SELECT
    T.*
INTO
    #Copy
FROM
    YourTable AS T
WHERE
    1 = 0

回答2:

The answer for both questions is simple NO.

Although, I agree with you that T-SQL should change in this way.

In the first case, it means having a command to clone a table structure.

Of course, there is a possibility to make your own T-SQL extension by using SQLCLR.