Sql – ny difference between GROUP BY and DISTINCT

distinctgroup-bysql

I learned something simple about SQL the other day:

SELECT c FROM myTbl GROUP BY C

Has the same result as:

SELECT DISTINCT C FROM myTbl

What I am curious of, is there anything different in the way an SQL engine processes the command, or are they truly the same thing?

I personally prefer the distinct syntax, but I am sure it's more out of habit than anything else.

EDIT: This is not a question about aggregates. The use of GROUP BY with aggregate functions is understood.

Best Answer

MusiGenesis' response is functionally the correct one with regard to your question as stated; the SQL Server is smart enough to realize that if you are using "Group By" and not using any aggregate functions, then what you actually mean is "Distinct" - and therefore it generates an execution plan as if you'd simply used "Distinct."

However, I think it's important to note Hank's response as well - cavalier treatment of "Group By" and "Distinct" could lead to some pernicious gotchas down the line if you're not careful. It's not entirely correct to say that this is "not a question about aggregates" because you're asking about the functional difference between two SQL query keywords, one of which is meant to be used with aggregates and one of which is not.

A hammer can work to drive in a screw sometimes, but if you've got a screwdriver handy, why bother?

(for the purposes of this analogy, Hammer : Screwdriver :: GroupBy : Distinct and screw => get list of unique values in a table column)

Related Solutions

Sql – Difference between EXISTS and IN in SQL

The exists keyword can be used in that way, but really it's intended as a way to avoid counting:

--this statement needs to check the entire table
select count(*) from [table] where ...

--this statement is true as soon as one match is found
exists ( select * from [table] where ... )

This is most useful where you have if conditional statements, as exists can be a lot quicker than count.

The in is best used where you have a static list to pass:

 select * from [table]
 where [field] in (1, 2, 3)

When you have a table in an in statement it makes more sense to use a join, but mostly it shouldn't matter. The query optimiser should return the same plan either way. In some implementations (mostly older, such as Microsoft SQL Server 2000) in queries will always get a nested join plan, while join queries will use nested, merge or hash as appropriate. More modern implementations are smarter and can adjust the plan even when in is used.

Sql – Insert into … values ( SELECT … FROM … )

Try:

INSERT INTO table1 ( column1 )
SELECT  col1
FROM    table2

This is standard ANSI SQL and should work on any DBMS

It definitely works for:

Oracle
MS SQL Server
MySQL
Postgres
SQLite v3
Teradata
DB2
Sybase
Vertica
HSQLDB
H2
AWS RedShift
SAP HANA
Google Spanner

Best Answer

Related Solutions

Sql – Difference between EXISTS and IN in SQL

Sql – Insert into … values ( SELECT … FROM … )

Related Topic