SELECT statement
Introduction#
In SQL, SELECT
statements return sets of results from data collections like tables or views. SELECT
statements can be used with various other clauses like WHERE
, GROUP BY
, or ORDER BY
to further refine the desired results.
Basic SELECT from table
Select all columns from some table (system table in this case):
SELECT *
FROM sys.objects
Or, select just some specific columns:
SELECT object_id, name, type, create_date
FROM sys.objects
Filter rows using WHERE clause
WHERE clause filters only those rows that satisfy some condition:
SELECT *
FROM sys.objects
WHERE type = 'IT'
Sort results using ORDER BY
ORDER BY clause sorts rows in the returned result set by some column or expression:
SELECT *
FROM sys.objects
ORDER BY create_date
Group result using GROUP BY
GROUP BY clause groups rows by some value:
SELECT type, count(*) as c
FROM sys.objects
GROUP BY type
You can apply some function on each group (aggregate function) to calculate sum or count of the records in the group.
type | c |
---|---|
SQ | 3 |
S | 72 |
IT | 16 |
PK | 1 |
U | 5 |
Filter groups using HAVING clause
HAVING clause removes groups that do not satisfy condition:
SELECT type, count(*) as c
FROM sys.objects
GROUP BY type
HAVING count(*) < 10
type | c |
---|---|
SQ | 3 |
PK | 1 |
U | 5 |
Returning only first N rows
TOP clause returns only first N rows in the result:
SELECT TOP 10 *
FROM sys.objects
Pagination using OFFSET FETCH
OFFSET FETCH clause is more advanced version of TOP. It enables you to skip N1 rows and take next N2 rows:
SELECT *
FROM sys.objects
ORDER BY object_id
OFFSET 50 ROWS FETCH NEXT 10 ROWS ONLY
You can use OFFSET without fetch to just skip first 50 rows:
SELECT *
FROM sys.objects
ORDER BY object_id
OFFSET 50 ROWS
SELECT without FROM (no data souce)
SELECT statement can be executed without FROM clause:
declare @var int = 17;
SELECT @var as c1, @var + 2 as c2, 'third' as c3
In this case, one row with values/results of expressions are returned.