SELECT list - Amazon Redshift
SyntaxParametersUsage notesExamples

SELECT list

The SELECT list names the columns, functions, and expressions that you want the query to return. The list represents the output of the query.

For more information about SQL functions, see SQL functions reference. For more information about expressions, see Conditional expressions.

Syntax

SELECT
[ TOP number ]
[ ALL | DISTINCT ] * | expression [ AS column_alias ] [, ...]

Parameters

TOP number

TOP takes a positive integer as its argument, which defines the number of rows that are returned to the client. The behavior with the TOP clause is the same as the behavior with the LIMIT clause. The number of rows that is returned is fixed, but the set of rows isn't. To return a consistent set of rows, use TOP or LIMIT in conjunction with an ORDER BY clause.

ALL

A redundant keyword that defines the default behavior if you don't specify DISTINCT. SELECT ALL * means the same as SELECT * (select all rows for all columns and retain duplicates).

DISTINCT

Option that eliminates duplicate rows from the result set, based on matching values in one or more columns.

Note

If your application allows invalid foreign keys or primary keys, it can cause queries to return incorrect results. For example, a SELECT DISTINCT query might return duplicate rows if the primary key column doesn't contain all unique values. For more information, see Defining table constraints.

* (asterisk)

Returns the entire contents of the table (all columns and all rows).

expression

An expression formed from one or more columns that exist in the tables referenced by the query. An expression can contain SQL functions. For example: 

avg(datediff(day, listtime, saletime))
AS column_alias

A temporary name for the column that is used in the final result set. The AS keyword is optional. For example: 

avg(datediff(day, listtime, saletime)) as avgwait

If you don't specify an alias for an expression that isn't a simple column name, the result set applies a default name to that column.

Note

The alias is recognized right after it is defined in the target list. You can use an alias in other expressions defined after it in the same target list. The following example illustrates this. 


select clicks / impressions as probability, round(100 * probability, 1) as percentage from raw_data;

The benefit of the lateral alias reference is you don't need to repeat the aliased expression when building more complex expressions in the same target list. When Amazon Redshift parses this type of reference, it just inlines the previously defined aliases. If there is a column with the same name defined in the FROM clause as the previously aliased expression, the column in the FROM clause takes priority. For example, in the above query if there is a column named 'probability' in table raw_data, the 'probability' in the second expression in the target list refers to that column instead of the alias name 'probability'.

Usage notes

TOP is a SQL extension; it provides an alternative to the LIMIT behavior. You can't use TOP and LIMIT in the same query.

Examples

The following example returns 10 rows from the SALES table. Though the query uses the TOP clause, it still returns an unpredictable set of rows because no ORDER BY clause is specified,

select top 10 *
from sales;

The following query is functionally equivalent, but uses a LIMIT clause instead of a TOP clause:

select *
from sales
limit 10;

The following example returns the first 10 rows from the SALES table using the TOP clause, ordered by the QTYSOLD column in descending order.

select top 10 qtysold, sellerid
from sales
order by qtysold desc, sellerid;

qtysold | sellerid
--------+----------
8 |      518
8 |      520
8 |      574
8 |      718
8 |      868
8 |     2663
8 |     3396
8 |     3726
8 |     5250
8 |     6216
(10 rows)

The following example returns the first two QTYSOLD and SELLERID values from the SALES table, ordered by the QTYSOLD column:

select top 2 qtysold, sellerid
from sales
order by qtysold desc, sellerid;

qtysold | sellerid
--------+----------
8 |      518
8 |      520
(2 rows)

The following example shows the list of distinct category groups from the CATEGORY table:

select distinct catgroup from category
order by 1;

catgroup
----------
Concerts
Shows
Sports
(3 rows)

--the same query, run without distinct
select catgroup from category
order by 1;

catgroup
----------
Concerts
Concerts
Concerts
Shows
Shows
Shows
Sports
Sports
Sports
Sports
Sports
(11 rows)

The following example returns the distinct set of week numbers for December 2008. Without the DISTINCT clause, the statement would return 31 rows, or one for each day of the month.

select distinct week, month, year
from date
where month='DEC' and year=2008
order by 1, 2, 3;

week | month | year
-----+-------+------
49 | DEC   | 2008
50 | DEC   | 2008
51 | DEC   | 2008
52 | DEC   | 2008
53 | DEC   | 2008
(5 rows)