Pull columns from series of joins in SQL

Question

I am kind of stuck at one problem at my job where I need to pull 2 cols from base table and 1 column from a series of joins.

Please note that, I can not provide real data so I am using dummy column/table names and there are 100s of columns in real project.

Select A.Name,B.Age, D.Sal 
From A Left join B on A.iD=B.id and B.Date=CURRENT_DATE
(/* join A and B table and return distinct column which is B.XYZ) 
inner join C on C.iD=B.XYZ  
(/* join B and C take C.YYY column for next join */)
inner join D on D.id=C.YYY  
(/* Take out the D.Sal column from this join */) where A.Dept='IT'

I have written this query but it is taking forever to run because B.XYZ column has a lot of duplicates. how can I get distinct of B.XYZ column from that join.

Answer 1

For Joining Table B, you first get a distinct table of the columns you need from B then join.

SELECT 
  A.Name,
  B.Age, 
  D.Sal 
From A 
LEFT JOIN ( -- Instead of all cols (*), just id, Date, Age and xyz might do
  SELECT DISTINCT * FROM B 
) B ON A.iD = B.id AND B.Date = CURRENT_DATE
    --(/* join A and B table and return distinct column which is B.XYZ */) 
INNER JOIN C ON C.iD = B.XYZ  
    --(/*join B and C take C.YYY column for next join */)
INNER JOIN D ON D.id = C.YYY  
    --(/* Take out the D.Sal column from this join */)
WHERE A.Dept='IT'

Answer 2

You say you get the same rows multifold, because for a b.id, date and age you get the same xyz more than once, or so I understand it.

One option is to join with a subquery that gets the distinct data:

SELECT a.name, b.age, d.sal 
FROM a 
LEFT JOIN 
(
  SELECT DISTINCT id, date, age, xyz FROM b
) dist_b ON dist_b.id = a.id and dist_b.date = CURRENT_DATE
INNER JOIN c ON c.id = dist_b.xyz  
INNER JOIN d ON d.id = c.yyy  
WHERE a.dept = 'IT';

Of course you can even move the date condition inside the subquery:

SELECT a.name, b.age, d.sal 
FROM a 
LEFT JOIN 
(
  SELECT DISTINCT id, age, xyz FROM b WHERE date = CURRENT_DATE
) dist_b ON dist_b.id = a.id
INNER JOIN c ON c.id = dist_b.xyz  
INNER JOIN d ON d.id = c.yyy  
WHERE a.dept = 'IT';

Your LEFT OUTER JOIN doesn't work by the way. As you are inner joining the following tables, a match must exists, so your outer join becomes an inner join. For the outer join to work you would have to outer join the following tables, too.