A common mistake with DISTINCT is to code the DISTINCT statement before the COUNT statement. : Count « Aggregate Functions « SQL Server / T-SQL Tutorial

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SQL Server / T-SQL Tutorial » Aggregate Functions » Count 
9. 3. 5. A common mistake with DISTINCT is to code the DISTINCT statement before the COUNT statement.
Because each row counted is already unique, DISTINCT will have no impact.
5>
6>
7CREATE TABLE CD (
8>      CD_ID              integer  NOT NULL PRIMARY KEY,
9>      CD_Title           varchar(40),
10>      Composer_ID        integer  NOT NULL,
11>      Classif_ID         integer  NOT NULL,
12>      SalesPrice        money,
13>      AverageCost       money)
14> GO
1INSERT into CD VALUES(2000,"John",100,1,16.99,6.99)
2INSERT into CD VALUES(2001,"Chicago 16",107,1,14.99,5.99)
3INSERT into CD VALUES(2002,"Chicago 17",107,1,14.99,5.99)
4INSERT into CD VALUES(2003,"Chicago 18",107,1,14.99,5.99)
5INSERT into CD VALUES(2004,"Greatest Hits",107,1,16.99,7.99)
6INSERT into CD VALUES(2005,"Midnight",101,3,14.99,5.99)
7INSERT into CD VALUES(2006,"Mode",115,3,14.99,5.99)
8INSERT into CD VALUES(2007,"Ultra",115,3,15.99,5.99)
9INSERT into CD VALUES(2008,"Mindcrime",102,4,14.99,5.99)
10INSERT into CD VALUES(2009,"Empire",102,4,14.99,5.99)
11INSERT into CD VALUES(2010,"Land",102,4,12.99,4.99)
12INSERT into CD VALUES(2011,"Night",103,4,11.99,3.99)
13INSERT into CD VALUES(2012,"Pyromania",103,4,14.99,5.99)
14INSERT into CD VALUES(2013,"Hysteria",103,4,14.99,5.99)
15INSERT into CD VALUES(2014,"Hits",103,4,13.99,4.99)
16INSERT into CD VALUES(2015,"Hits 2",104,2,15.99,6.99)
17INSERT into CD VALUES(2016,"Greatest",105,2,14.99,5.99)
18INSERT into CD VALUES(2017,"Hits 3",106,1,13.99,5.99)
19INSERT into CD VALUES(2018,"Deep",108,1,12.99,2.99)
20INSERT into CD VALUES(2019,"Turning",109,1,14.99,5.99)
21INSERT into CD VALUES(2020,"TheHits",109,1,16.99,7.99)
22INSERT into CD VALUES(2021,"Cars",110,1,9.99,3.99)
23INSERT into CD VALUES(2022,"Anthology",110,1,25.99,11.99)
24INSERT into CD VALUES(2023,"City",110,1,14.99,5.99)
25INSERT into CD VALUES(2024,"Rick",111,1,11.99,2.99)
26INSERT into CD VALUES(2025,"Live",112,1,19.99,8.99)
27INSERT into CD VALUES(2026,"Pat",113,1,16.99,6.99)
28INSERT into CD VALUES(2027,"Big",114,1,14.99,5.99)
29INSERT into CD VALUES(2028,"Hurting",114,1,11.99,3.99)
30INSERT into CD VALUES(2029,"Vol 1",116,1,9.99,2.99)
31INSERT into CD VALUES(2030,"Vol 2",116,1,9.99,2.99)
32> GO

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1>
2>
3SELECT DISTINCT COUNT(Classif_IDfrom CD
4> GO

-----------
         31

(rows affected)
1>
2> drop table CD;
3> GO
9. 3. Count
9. 3. 1. COUNT returns the total count of records in a column or the count for DISTINCT (unique) values selected by the query.
9. 3. 2. COUNT(*) returns the total count of all records (rows) in the column, including NULL values of selected records by the query.
9. 3. 3. Count function with column alias
9. 3. 4. Counting unique values.
9. 3. 5. A common mistake with DISTINCT is to code the DISTINCT statement before the COUNT statement.
9. 3. 6. Counting values including NULL values.
9. 3. 7. Using ISNULL to count all values including NULLs.
9. 3. 8. Count table row in dynamic sql
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