left outer join - what's the difference? - oracle

Question

I sometimes get confused with LEFT JOINS and OR.
Here in the below two code samples, there is only one difference. There is a bracket in the WHERE clause in one sample and no bracket in other sample.
Obviously, I am getting different results. I want to understand what's happening.
Thanks.
SELECT FROM TABLE_A as A
LEFT OUTER JOIN TABLE_B as B
ON
A.col1 = B.col1 AND
A.col2 = B.col2 AND
A.col3 = B.col3
WHERE
B.col1 IS NULL OR B.col2 IS NULL OR B.col3 IS NULL AND
B.col4= 'R' AND
B.col5 != 'DR'
SELECT FROM TABLE_A as A
LEFT OUTER JOIN TABLE_B as B
ON
A.col1 = B.col1 AND
A.col2 = B.col2 AND
A.col3 = B.col3
WHERE
(B.col1 IS NULL OR B.col2 IS NULL OR B.col3 IS NULL) AND
B.col4= 'R' AND
B.col5 != 'DR'

Answer 1

(B.col1 IS NULL OR B.col2 IS NULL OR B.col3 IS NULL)
In second case, first the expression in the bracket is evaluated and then the following "AND" operators are applied. So the results if above bracket expression is ANDed with B.col4= 'R' and then it is ANDed with B.col5 != 'DR'
B.col1 IS NULL OR B.col2 IS NULL OR B.col3 IS NULL AND
B.col4= 'R' AND
B.col5 != 'DR'
In first case, it is evaluated like following:
(B.col1 IS NULL) OR (B.col2 IS NULL) OR (B.col3 IS NULL AND B.col4= 'R' AND B.col5 != 'DR')

Answer 2

The AND operator has higher precedence than OR, so your first WHERE clause can be written as this:
WHERE
B.col1 IS NULL OR B.col2 IS NULL OR (B.col3 IS NULL AND
B.col4= 'R' AND
B.col5 != 'DR')
But this may not be what you intended, if the second version of your WHERE clause is any evidence. If you want to check for either of the three columns being NULL ANDed with the other two conditions then you should use parentheses.

Answer 3

I think you've gotten very confused about left joins as well as the AND/OR operator precedence.
In your first query, the order is applied left to right, with ANDS being applied first.
That means that your first query is the equivalent of:
SELECT *
FROM table_a a
LEFT OUTER JOIN table_b b ON (a.col1 = b.col1
AND a.col2 = b.col2
AND a.col3 = b.col3)
WHERE b.col1 IS NULL
OR b.col2 IS NULL
OR (b.col3 IS NULL
AND b.col4 = 'R'
AND b.col5 != 'DR');
However, your second query is checking for nulls in any of the three join conditions (which would either be all null, or all not null, since you haven't catered for null values in your join conditions) as well as checking for the presence of values in the remaining two columns. Which doesn't make sense - if col1, col2 and col3 are null, it means that table_b row isn't part of the join, so col4 and col5 would be null as well.
Instead, what I think you meant to do was to make the and b.col4 = 'R' and b.col5 != 'DR' part of the left outer join conditions - ie. only join the rows from table_b where col4 = 'R' and col5 != 'DR' - like so:
WITH table_a AS (SELECT 1 col1, 1 col2, 1 col3 FROM dual UNION ALL
SELECT 2 col1, 2 col2, 2 col3 FROM dual UNION ALL
SELECT 3 col1, 3 col2, 3 col3 FROM dual),
table_b AS (SELECT 1 col1, 1 col2, 1 col3, 'R' col4, 'DP' col5 FROM dual UNION ALL
SELECT 2 col1, 2 col2, 2 col3, 'R' col4, 'DR' col5 FROM dual)
SELECT *
FROM table_a a
LEFT OUTER JOIN table_b b ON (a.col1 = b.col1
AND a.col2 = b.col2
AND a.col3 = b.col3
AND b.col4 = 'R'
AND b.col5 != 'DR');
COL1 COL2 COL3 COL1 COL2 COL3 COL4 COL5
---------- ---------- ---------- ---------- ---------- ---------- ---- ----
1 1 1 1 1 1 R DP
2 2 2
3 3 3

Answer 4

Your second query is not a LEFT OUTER JOIN:
SELECT *
FROM TABLE_A A
LEFT OUTER JOIN TABLE_B B
ON ( A.col1 = B.col1
AND A.col2 = B.col2
AND A.col3 = B.col3 )
WHERE ( B.col1 IS NULL OR B.col2 IS NULL OR B.col3 IS NULL )
AND B.col4= 'R'
AND B.col5 != 'DR'
The WHERE clause will only accept rows where B.col4 and B.col5 are NOT NULL and this will only ever be the case when the tables are joined and this means the join condition is effectively an INNER JOIN.
However, the conditions ( B.col1 IS NULL OR B.col2 IS NULL OR B.col3 IS NULL ) and ( A.col1 = B.col1 AND A.col2 = B.col2 AND A.col3 = B.col3 ) can never be fulfilled simultaneously since a NULL value cannot equal any other value (including another NULL). So the query should never return any rows.
The first query is:
SELECT *
FROM TABLE_A A
LEFT OUTER JOIN TABLE_B B
ON ( A.col1 = B.col1
AND A.col2 = B.col2
AND A.col3 = B.col3 )
WHERE B.col1 IS NULL
OR B.col2 IS NULL
OR ( B.col3 IS NULL AND B.col4= 'R' AND B.col5 != 'DR' )
This will return rows when B.col1 IS NULL OR B.col2 IS NULL but will never match on ( B.col3 IS NULL AND B.col4= 'R' AND B.col5 != 'DR' ) since the join condition will enforce the either B.col3 is not NULL (when there is a match in the LEFT OUTER JOIN) or B.col4 and B.col5 are both NULL (when there is no match in the LEFT OUTER JOIN).
What you might actually be trying to achieve (although it is unclear from your question) is:
SELECT *
FROM TABLE_A A
LEFT OUTER JOIN TABLE_B B
ON ( A.col1 = B.col1
AND A.col2 = B.col2
AND A.col3 = B.col3
AND B.col4 = 'R'
AND B.col5 != 'DR' )