Why MySQL's LEFT JOIN is returning “NULL” records when with WHERE clause?

Question

Today I\'ve tried some more complex MySQL queries and I\'ve noticed that MySQL\'s LEFT JOIN is not working with WHERE clause. I mean, it does return some records but it does not

Answer 1

"The LEFT JOIN keyword returns all rows from the left table (table1), with the matching rows in the right table (table2). The result is NULL in the right side when there is no match."

The above quote is from w3schools.com

However, when you place a WHERE clause on a LEFT JOIN, SQL now treats that as an INNER JOIN and:

"The INNER JOIN keyword selects all rows from both tables as long as there is a match between the columns in both tables."

Also quoted from w3schools.com

TLDR;

The introduction of the WHERE clause to a LEFT OUTER JOIN gives the join the behavior of an INNER JOIN

Answer 2

A left join condition and where condition filter are not both same. Data is filtered by the where clause after the physical join is done. if you look a left join it will normally return every row from your left table, but once you have a where clause, it will filter the output of the join so the result is like an inner join. You will want to focus on the two diagrams on the left side of the image below.

Answer 3

First Case :

After joining, the records are getting filtered by the (WHERE clause). So only 1 result.

Second Case (when you replace WHERE with AND)

Will return all join entries + entries satisfied in second condition (t2.rank != 17). That is why here you get 2 records ( one from join + another from AND clause)

Answer 4

The solution to this question becomes quite intuitive once one is aware of the execution order or Logical Query Processing Phases of the SQL statement. The order is: -

1. FROM
2. ON
3. OUTER
4. WHERE
5. GROUP BY
6. CUBE | ROLLUP
7. HAVING
8. SELECT
9. DISTINCT
10. ORDER BY
11. TOP

As ON is performed before the OUTER(LEFT/RIGHT) part(adding NULL valued rows) of the JOIN, the 1st case has rows with NULL values in rank column. In the 2nd case the rows get filtered out based on the values of the rank column after the OUTER JOIN(LEFT JOIN here) is performed. Hence, the rows with NULL values in the rank column are filtered out.

One very important thing that can be noticed in your SQL query is the comparison with NULL This area requires special attention as the NULL values when with NULL/NON-NULL values using the normal arithmetic operators result NULL(it is neither TRUE nor FALSE) because NULL means no value is available for comparison. This behavior is defined in the ANSI SQL-92 standard.(This can be overridden by turning ansi null(actual name may vary) parameter off in some SQL processors) So, your SQL query with where clause filters out rows with NULL value in rank column which might seem counter-intuitive due to "17 != NULL" seems TRUE ex-

NULL = NULL results NULL/UNKNOWN

17 = NULL results NULL/UNKNOWN

17 != NULL results NULL?UNKNOWN

Some interesting posts/blogs for reference are:

http://blog.sqlauthority.com/2009/04/06/sql-server-logical-query-processing-phases-order-of-statement-execution/ http://blog.sqlauthority.com/2009/03/15/sql-server-interesting-observation-of-on-clause-on-left-join-how-on-clause-effects-resultset-in-left-join/ http://www.xaprb.com/blog/2006/05/18/why-null-never-compares-false-to-anything-in-sql/