SQL: Ignore Condition in WHERE clause - sql

Question

I am confused on how to exclude one of the conditions in WHERE clause if it evaluates to NULL
SELECT TB1.COL3
FROM TB1, TB2
WHERE TB1.COL1 = TB2.Col1
AND TB2.COL1 = '12345'
AND (TB2.COL3 = (SELECT MIN(TB3.COL4)
FROM TB3
WHERE COL1 = TB2.COL1
AND COL2 in ('A', 'B')
AND COL4 IN (SELECT COL3 FROM TB4
WHERE COL1 = TB4.COL1)))
AND ROWNUM = 1;
How can I modify the above query to ignore the "AND condition TB2.COL3" if the following condition SELECT MIN(TB3.COL4) FROM TB3 evaluates to some value except NULL. If NULL the above query should be executed as below:
SELECT TB1.COL3
FROM TB1, TB2
WHERE TB1.COL1 = TB2.Col1
AND TB2.COL1 = '12345'
AND ROWNUM = 1;

Answer 1

I think you could pull this off with a coalesce:
SELECT TB1.COL3
FROM TB1, TB2
WHERE TB1.COL1 = TB2.Col1
AND TB2.COL1 = '12345'
AND (TB2.COL3 = COALESCE((SELECT MIN(TB3.COL4)
FROM TB3
WHERE COL1 = TB2.COL1
AND COL2 in ('A', 'B')
AND COL4 IN (SELECT COL3 FROM TB2
WHERE COL1 = TB2.COL1)), TB2.COL3))
AND ROWNUM = 1;
Here we are just wrapping that SELECT statement in COALESCE which, if it returns null, will grab the second parameter's value instead, which is the same column we are comparing. Since TB2.COL3 = TB2.COL3 is obviously TRUE then this filter will not have any impact on the result set.
Also, it's generally recommended that you stop using comma's in your FROM clause and use proper JOIN syntax. It's been around for over 2 decades now:
SELECT
TB1.COL3
FROM
TB1
INNER JOIN TB2
ON TB1.COL1 = TB2.Col1
WHERE
TB2.COL1 = '12345'
AND
(
TB2.COL3 = COALESCE
(
(
SELECT
MIN(TB3.COL4)
FROM
TB3
WHERE
COL1 = TB2.COL1
AND COL2 in ('A', 'B')
AND COL4 IN
(
SELECT
COL3
FROM
TB2
WHERE
COL1 = TB2.COL1
)
),
TB2.COL3
)
)
AND ROWNUM = 1;