加入表格中的最大日期

Question

SELECT COL1,
    COL2,
    COL3
FROM TABLE1,
    TABLE2,
    TABLE3,
    TABLE4
WHERE TABLE1.KEY1 = TABLE2.KEY1
    AND TABLE2.KEY = TABLE3.KEY
    AND TABLE2.FILTER = 'Y'
    AND TABLE3.FILTER = 'Y'
    AND TABLE2.KEY = TABLE3.KEY
    AND TABLE3.KEY = TABLE4.KEY

I have a similar query and I need to do modification, in a table 3 there is a date column and I need to pick highest day value row for joining. Lets say there are 4 rows from table number 3 which are getting satisfied for join, I need to pick highest date row out of those 4 for joining purpose and then show the result.

Hope question is clear. Database oracle 10g

Answer 1

Try something like this query.

SELECT
    COL1,
    COL2,
    COL3,
    T33.*

FROM TABLE1

JOIN TABLE2 ON TABLE1.KEY1 = TABLE2.KEY1

JOIN TABLE4 ON TABLE2.KEY = TABLE4.KEY

JOIN

(
    SELECT MAX(T.Day) as DT, T.KEY
    FROM TABLE3 T
    WHERE T.FILTER = 'Y'
    GROUP BY T.KEY
) T3 on TABLE4.KEY = T3.KEY

JOIN TABLE3 T33 ON T3.KEY = T33.KEY AND T3.DT = T33.Day

WHERE

TABLE2.FILTER = 'Y'

The main idea is that instead of
joining to TABLE3 you do this:

SELECT MAX(T.Day) as DT, T.KEY
FROM TABLE3 T
WHERE T.FILTER = 'Y'
GROUP BY T.KEY

give that table/recordset a name and join to it instead.

Then you can join again to the original TABLE3 (see T33 )
to pull all the other needed columns from TABLE3 which are
not present in T3 .

You can work out the other details, I think.

Answer 2

To minimally modify your current query, you can add a condition in your WHERE clause

AND TABLE3.DATE = (SELECT MAX(DATE) FROM TABLE3 WHERE TABLE3.FILTER = 'Y')

Although in the future I recommend using explicit JOINS.

SELECT COL1,
    COL2,
    COL3
FROM TABLE1 
    INNER JOIN TABLE2 ON TABLE1.KEY1 = TABLE2.KEY1
    INNER JOIN TABLE3 ON TABLE2.KEY = TABLE3.KEY
    INNER JOIN TABLE4 ON TABLE3.KEY = TABLE4.KEY
WHERE 
    TABLE2.FILTER = 'Y'
    AND TABLE3.FILTER = 'Y'
    AND TABLE3.DATE = (SELECT MAX(DATE) FROM TABLE3 WHERE TABLE3.FILTER = 'Y')