SELECT Clause (Transact-SQL)

บทความ
02/28/2023

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance SQL analytics endpoint in Microsoft Fabric Warehouse in Microsoft Fabric

Specifies the columns to be returned by the query.

Transact-SQL syntax conventions

Syntax

SELECT [ ALL | DISTINCT ]  
[ TOP ( expression ) [ PERCENT ] [ WITH TIES ] ]   
<select_list>   
<select_list> ::=   
    {   
      *   
      | { table_name | view_name | table_alias }.*   
      | {  
          [ { table_name | view_name | table_alias }. ]  
               { column_name | $IDENTITY | $ROWGUID }   
          | udt_column_name [ { . | :: } { { property_name | field_name }   
            | method_name ( argument [ ,...n] ) } ]  
          | expression  
         }  
        [ [ AS ] column_alias ]   
      | column_alias = expression   
    } [ ,...n ]

Arguments

ALL
Specifies that duplicate rows can appear in the result set. ALL is the default.

DISTINCT
Specifies that only unique rows can appear in the result set. Null values are considered equal for the purposes of the DISTINCT keyword.

TOP (expression ) [ PERCENT ] [ WITH TIES ]
Indicates that only a specified first set or percent of rows will be returned from the query result set. expression can be either a number or a percent of the rows.

For backward compatibility, using the TOP expression without parentheses in SELECT statements is supported, but we do not recommend it. For more information, see TOP (Transact-SQL).

<select_list> The columns to be selected for the result set. The select list is a series of expressions separated by commas. The maximum number of expressions that can be specified in the select list is 4096.

*
Specifies that all columns from all tables and views in the FROM clause should be returned. The columns are returned by table or view, as specified in the FROM clause, and in the order in which they exist in the table or view.

table_name | view_name | table_alias.*
Limits the scope of the * to the specified table or view.

column_name
Is the name of a column to return. Qualify column_name to prevent an ambiguous reference, such as occurs when two tables in the FROM clause have columns with duplicate names. For example, the SalesOrderHeader and SalesOrderDetail tables in the AdventureWorks2022 database both have a column named ModifiedDate. If the two tables are joined in a query, the modified date of the SalesOrderDetail entries can be specified in the select list as SalesOrderDetail.ModifiedDate.

expression
Is a constant, function, any combination of column names, constants, and functions connected by an operator or operators, or a subquery.

$IDENTITY
Returns the identity column. For more information, see IDENTITY (Property) (Transact-SQL), ALTER TABLE (Transact-SQL), and CREATE TABLE (Transact-SQL).

If more than one table in the FROM clause has a column with the IDENTITY property, $IDENTITY must be qualified with the specific table name, such as T1.$IDENTITY.

$ROWGUID
Returns the row GUID column.

If there is more than one table in the FROM clause with the ROWGUIDCOL property, $ROWGUID must be qualified with the specific table name, such as T1.$ROWGUID.

udt_column_name
Is the name of a common language runtime (CLR) user-defined type column to return.

Note

SQL Server Management Studio returns user-defined type values in binary representation. To return user-defined type values in string or XML format, use CAST or CONVERT.

{ . | :: }
Specifies a method, property, or field of a CLR user-defined type. Use . for an instance (nonstatic) method, property, or field. Use :: for a static method, property, or field. To invoke a method, property, or field of a CLR user-defined type, you must have EXECUTE permission on the type.

property_name
Is a public property of udt_column_name.

field_name
Is a public data member of udt_column_name.

method_name
Is a public method of udt_column_name that takes one or more arguments. method_name cannot be a mutator method.

The following example selects the values for the Location column, defined as type point, from the Cities table, by invoking a method of the type called Distance:

CREATE TABLE dbo.Cities (  
     Name VARCHAR(20),  
     State VARCHAR(20),  
     Location POINT);  
GO  
DECLARE @p POINT (32, 23), @distance FLOAT;  
GO  
SELECT Location.Distance (@p)  
FROM Cities;

column_alias
Is an alternative name to replace the column name in the query result set. For example, an alias such as Quantity, or Quantity to Date, or Qty can be specified for a column named quantity.

Aliases are used also to specify names for the results of expressions, for example:

USE AdventureWorks2022;  
GO  
SELECT AVG(UnitPrice) AS [Average Price]  
FROM Sales.SalesOrderDetail;

column_alias can be used in an ORDER BY clause. However, it cannot be used in a WHERE, GROUP BY, or HAVING clause. If the query expression is part of a DECLARE CURSOR statement, column_alias cannot be used in the FOR UPDATE clause.

Remarks

The length of data returned for text or ntext columns that are included in the select list is set to the smallest value of the following: the actual size of the text column, the default TEXTSIZE session setting, or the hard-coded application limit. To change the length of returned text for the session, use the SET statement. By default, the limit on the length of text data returned with a SELECT statement is 4,000 bytes.

The SQL Server Database Engine raises exception 511 and rolls back the current running statement if either of the following behavior occurs:

The SELECT statement produces a result row or an intermediate work table row exceeding 8,060 bytes.
The DELETE, INSERT, or UPDATE statement tries an action on a row exceeding 8,060 bytes.

An error occurs if no column name is specified to a column created by a SELECT INTO or CREATE VIEW statement.

แชร์ผ่าน

SELECT Clause (Transact-SQL)

Syntax

Arguments

Remarks

See Also

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