Remove subquery in Postgresql

Question

Remove subquery in Postgresql

WITH Dept(DName) AS (
   VALUES ('D1'), ('D2'), ('D3')
) ,
Emp(DName, EName, Age, Loc) AS (
     VALUES ('D1','E11',20,'L1'), ('D1','E12',25,'L2'),('D2','E21',28,'L1')
) 
SELECT DName,

  (SELECT avg(Age)
   FROM Emp
   WHERE Loc = 'L1'
     AND DName = d.DName) AS "L1 Avg",

  (SELECT avg(Age)
   FROM Emp
   WHERE Loc = 'L2'
     AND DName = d.DName) AS "L2 Avg"
FROM Dept d
LEFT JOIN Emp USING (DName)
GROUP BY DName

The result of this query:

"D1";20.0000000000000000;25.0000000000000000
"D2";28.0000000000000000;
"D3";;

Is there a way to refactor the query by removing the subquery and replacing it with a better construct in Postgresql?

+3

sql join postgresql subquery

Gopal 12 Feb 13 at 7:31

source to share

1 answer

a_horse_with_no_name · Accepted Answer · 2013-02-12T08:15:26+0000

SELECT DName,
       avg(case when loc = 'L1' then age else null end) as "L1 Avg",
       avg(case when loc = 'L2' then age else null end) as "L2 Avg"
FROM Dept d
LEFT JOIN Emp USING (DName)
GROUP BY DName

Remove subquery in Postgresql

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