table (Transact-SQL)

APPLIES TO: yesSQL Server yesAzure SQL Database noAzure SQL Data Warehouse noParallel Data Warehouse

Is a special data type used to store a result set for processing at a later time. table is primarily used for temporarily storing a set of rows that are returned as the table-valued function result set. Functions and variables can be declared to be of type table. table variables can be used in functions, stored procedures, and batches. To declare variables of type table, use DECLARE @local_variable.

Applies to: SQL Server (SQL Server 2008 through SQL Server 2017), Azure SQL Database.

Topic link icon Transact-SQL Syntax Conventions

Syntax

table_type_definition ::=   
    TABLE ( { <column_definition> | <table_constraint> } [ ,...n ] )   
  
<column_definition> ::=   
    column_name scalar_data_type   
    [ COLLATE <collation_definition> ]   
    [ [ DEFAULT constant_expression ] | IDENTITY [ ( seed , increment ) ] ]   
    [ ROWGUIDCOL ]   
    [ column_constraint ] [ ...n ]   
  
 <column_constraint> ::=   
    { [ NULL | NOT NULL ]   
    | [ PRIMARY KEY | UNIQUE ]   
    | CHECK ( logical_expression )   
    }   
  
<table_constraint> ::=   
     { { PRIMARY KEY | UNIQUE } ( column_name [ ,...n ] )  
     | CHECK ( logical_expression )   
     }   

Arguments

table_type_definition
Is the same subset of information that is used to define a table in CREATE TABLE. The table declaration includes column definitions, names, data types, and constraints. The only constraint types allowed are PRIMARY KEY, UNIQUE KEY, and NULL.
For more information about the syntax, see CREATE TABLE (Transact-SQL), CREATE FUNCTION (Transact-SQL), and DECLARE @local_variable (Transact-SQL).

collation_definition
Is the collation of the column that is made up of a Microsoft Windows locale and a comparison style, a Windows locale, and the binary notation, or a Microsoft SQL Server collation. If collation_definition isn't specified, the column inherits the collation of the current database. Or if the column is defined as a common language runtime (CLR) user-defined type, the column inherits the collation of the user-defined type.

Remarks

table Reference variables by name in a batch's FROM clause, as shown the following example:

SELECT Employee_ID, Department_ID FROM @MyTableVar;  

Outside a FROM clause, table variables must be referenced by using an alias, as shown in the following example:

SELECT EmployeeID, DepartmentID   
FROM @MyTableVar m  
JOIN Employee on (m.EmployeeID =Employee.EmployeeID AND  
   m.DepartmentID = Employee.DepartmentID);  

table variables provide the following benefits for small-scale queries that have query plans that don't change and when recompilation concerns are dominant:

  • A table variable behaves like a local variable. It has a well-defined scope. This variable is the function, stored procedure, or batch that it's declared in.
    Within its scope, a table variable can be used like a regular table. It may be applied anywhere a table or table expression is used in SELECT, INSERT, UPDATE, and DELETE statements. However, table can't be used in the following statement:
SELECT select_list INTO table_variable;

table variables are automatically cleaned up at the end of the function, stored procedure, or batch in which they're defined.

  • table variables that are used in stored procedures cause fewer stored procedure recompilations than when temporary tables are used when there are no cost-based choices that affect performance.
  • Transactions involving table variables last only for the duration of an update on the table variable. As such, table variables require less locking and logging resources.

Limitations and restrictions

Table variables don't have distribution statistics. They won't trigger recompiles. In many cases, the optimizer will build a query plan on the assumption that the table variable has no rows. For this reason, you should be cautious about using a table variable if you expect a larger number of rows (greater than 100). Temp tables may be a better solution in this case. For queries that join the table variable with other tables, use the RECOMPILE hint, which will cause the optimizer to use the correct cardinality for the table variable.

table variables aren't supported in the SQL Server optimizer's cost-based reasoning model. As such, they shouldn't be used when cost-based choices are required to achieve an efficient query plan. Temporary tables are preferred when cost-based choices are required. This plan typically includes queries with joins, parallelism decisions, and index selection choices.

Queries that modify table variables don't generate parallel query execution plans. Performance can be affected when large table variables, or table variables in complex queries, are modified. Consider using temporary tables instead in situations where table variables are modified. For more information, see CREATE TABLE (Transact-SQL). Queries that read table variables without modifying them can still be parallelized.

Indexes can't be created explicitly on table variables, and no statistics are kept on table variables. Starting with SQL Server 2014 (12.x), new syntax was introduced which allows you to create certain index types inline with the table definition. Using this new syntax, you can create indexes on table variables as part of the table definition. In some cases, performance may improve by using temporary tables instead, which provide full index support and statistics. For more information about temporary tables and inline index creation, see CREATE TABLE (Transact-SQL).

CHECK constraints, DEFAULT values, and computed columns in the table type declaration can't call user-defined functions.

Assignment operation between table variables isn't supported.

Because table variables have limited scope and aren't part of the persistent database, transaction rollbacks don't affect them.

Table variables can't be altered after creation.

Table variable deferred compilation

Table variable deferred compilation improves plan quality and overall performance for queries referencing table variables. During optimization and initial plan compilation, this feature will propagate cardinality estimates that are based on actual table variable row counts. This exact row count information will then be used for optimizing downstream plan operations.

Note

Table variable deferred compilation is a public preview feature in Azure SQL Database and SQL Server 2019 preview.

With table variable deferred compilation, compilation of a statement that references a table variable is deferred until the first actual execution of the statement. This deferred compilation behavior is identical to the behavior of temporary tables. This change results in the use of actual cardinality instead of the original one-row guess.

To enable the public preview of table variable deferred compilation, enable database compatibility level 150 for the database you're connected to when the query runs.

Table variable deferred compilation doesn't change any other characteristics of table variables. For example, this feature doesn't add column statistics to table variables.

Table variable deferred compilation doesn't increase recompilation frequency. Rather, it shifts where the initial compilation occurs. The resulting cached plan generates based on the initial deferred compilation table variable row count. The cached plan is reused by consecutive queries. It's reused until the plan is evicted or recompiled.

Table variable row count that is used for initial plan compilation represents a typical value might be different from a fixed row count guess. If it's different, downstream operations will benefit. Performance may not be improved by this feature if the table variable row count varies significantly across executions.

Disabling table variable deferred compilation without changing the compatibility level

Disable table variable deferred compilation at the database or statement scope while still maintaining database compatibility level 150 and higher. To disable table variable deferred compilation for all query executions originating from the database, execute the following example within the context of the applicable database:

ALTER DATABASE SCOPED CONFIGURATION SET DEFERRED_COMPILATION_TV = OFF;

To re-enable table variable deferred compilation for all query executions originating from the database, execute the following example within the context of the applicable database:

ALTER DATABASE SCOPED CONFIGURATION SET DEFERRED_COMPILATION_TV = ON;

You can also disable table variable deferred compilation for a specific query by assigning DISABLE_DEFERRED_COMPILATION_TV as a USE HINT query hint. For example:

DECLARE @LINEITEMS TABLE 
	(L_OrderKey INT NOT NULL,
	 L_Quantity INT NOT NULL
	);

INSERT @LINEITEMS
SELECT L_OrderKey, L_Quantity
FROM dbo.lineitem
WHERE L_Quantity = 5;

SELECT	O_OrderKey,
	O_CustKey,
	O_OrderStatus,
	L_QUANTITY
FROM	
	ORDERS,
	@LINEITEMS
WHERE	O_ORDERKEY	=	L_ORDERKEY
	AND O_OrderStatus = 'O'
OPTION (USE HINT('DISABLE_DEFERRED_COMPILATION_TV'));

Examples

A. Declaring a variable of type table

The following example creates a table variable that stores the values specified in the OUTPUT clause of the UPDATE statement. Two SELECT statements follow that return the values in @MyTableVar and the results of the update operation in the Employee table. Results in the INSERTED.ModifiedDate column differ from the values in the ModifiedDate column in the Employee table. This difference is because the AFTER UPDATE trigger, which updates the value of ModifiedDate to the current date, is defined on the Employee table. However, the columns returned from OUTPUT reflect the data before triggers are fired. For more information, see OUTPUT Clause (Transact-SQL).

USE AdventureWorks2012;  
GO  
DECLARE @MyTableVar table(  
    EmpID int NOT NULL,  
    OldVacationHours int,  
    NewVacationHours int,  
    ModifiedDate datetime);  
UPDATE TOP (10) HumanResources.Employee  
SET VacationHours = VacationHours * 1.25   
OUTPUT INSERTED.BusinessEntityID,  
       DELETED.VacationHours,  
       INSERTED.VacationHours,  
       INSERTED.ModifiedDate  
INTO @MyTableVar;  
--Display the result set of the table variable.  
SELECT EmpID, OldVacationHours, NewVacationHours, ModifiedDate  
FROM @MyTableVar;  
GO  
--Display the result set of the table.  
--Note that ModifiedDate reflects the value generated by an  
--AFTER UPDATE trigger.  
SELECT TOP (10) BusinessEntityID, VacationHours, ModifiedDate  
FROM HumanResources.Employee;  
GO  

B. Creating an inline table-valued function

The following example returns an inline table-valued function. It returns three columns ProductID, Name, and the aggregate of year-to-date totals by store as YTD Total for each product sold to the store.

USE AdventureWorks2012;  
GO  
IF OBJECT_ID (N'Sales.ufn_SalesByStore', N'IF') IS NOT NULL  
    DROP FUNCTION Sales.ufn_SalesByStore;  
GO  
CREATE FUNCTION Sales.ufn_SalesByStore (@storeid int)  
RETURNS TABLE  
AS  
RETURN   
(  
    SELECT P.ProductID, P.Name, SUM(SD.LineTotal) AS 'Total'  
    FROM Production.Product AS P   
    JOIN Sales.SalesOrderDetail AS SD ON SD.ProductID = P.ProductID  
    JOIN Sales.SalesOrderHeader AS SH ON SH.SalesOrderID = SD.SalesOrderID  
    JOIN Sales.Customer AS C ON SH.CustomerID = C.CustomerID  
    WHERE C.StoreID = @storeid  
    GROUP BY P.ProductID, P.Name  
);  
GO  

To invoke the function, run this query.

SELECT * FROM Sales.ufn_SalesByStore (602);  

See also

COLLATE (Transact-SQL)
CREATE FUNCTION (Transact-SQL)
User-Defined Functions
CREATE TABLE (Transact-SQL)
DECLARE @local_variable (Transact-SQL)
Use Table-Valued Parameters (Database Engine)
Query Hints (Transact-SQL)