MySQL Left Join Null Result

Mysql Left Join Null Result

Use:

   SELECT a.*, 
          COALESCE(u.username, 'User Not Found') AS `user` 
     FROM ARTICLES a
LEFT JOIN USERS u ON u.id = a.user_id
 ORDER BY articles.timestamp 

Documentation:

• COALESCE

The reason to choose COALESCE over IF or IFNULL is that COALESCE is ANSI standard, while the other methods are not reliably implemented over other databases. I would use CASE before I'd look at IF because again - CASE is ANSI standard, making it easier to port the query to other databases.

MySQL select rows where left join is null

You could use the following query:

SELECT  table1.id 
FROM    table1 
        LEFT JOIN table2 
            ON table1.id IN (table2.user_one, table2.user_two)
WHERE   table2.user_one IS NULL;

Although, depending on your indexes on table2 you may find that two joins performs better:

SELECT  table1.id 
FROM    table1 
        LEFT JOIN table2 AS t1
            ON table1.id = t1.user_one
        LEFT JOIN table2 AS t2
            ON table1.id = t2.user_two
WHERE   t1.user_one IS NULL
AND     t2.user_two IS NULL;

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

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.

MySQL Left Join not returning null values for joined table

You should not specify rYear in a WHERE clause. Those limit your results after the join. You should specify rYear in an ON clause to get back records with NULL from table B.

SELECT * from A
left join B 
on A.sid = B.sid 
AND (rYear = 2011 or rYear is null)
where (rCode = 1 Or rCode = 2 Or rCode = 3 Or rCode = 5)

LEFT JOIN on a nullable column. What is the behaviour?

If you have a LEFT JOIN and the right table returns nothing (NULL) all the fields belonging to the right table in the projection are simply NULL, but you still get your result from the left table. RIGHT JOIN has the opposite behavior and INNER JOIN will not return anything.

SELECT * FROM `left_table` LEFT JOIN `right_table`

NULL = NULL evaluates to UNKNOWN (which means “no, don’t join because I have no clue if we are allowed to.”) and the projection will only contain the results from the left table.

Of course there are ways to go around this problem:

SELECT *
FROM `left_table` AS `l`
    LEFT JOIN `right_table` AS `r`
        ON `r`.`id` <=> `l`.`id`

Now checks against NULL will work normally as you are used to (NULL <=> NULL is 1 and 'value' <=> NULL is 0). Also see the documentation for the equal to operator.

Left join on null values in MYSQL

First, I'll assume that your sample data set should have Abhisek 20000, not Abhisek 2000, because you don't get the results you're puzzling over if you use Abhisek 2000.

Second, the use of * is returning duplicate columns. (Yes, I know that one is called salary and the other is called Salary - but you're JOINing on that column. They're the same value. No need to return that value twice.)

Third, you don't seem to understand how a JOIN builds its results. And the "extra" rows you're seeing come from the fact that you're not JOINing on unique data. There are 2 rows in Salary that have a value of 10000 in the Salary column.

So your LEFT JOIN starts from the Salary table and tries to find a match in the Employee table for every record in the Salary column. This is how the rows get built from the JOIN:

1. It looks for any records where Salary.Salary = 10000 AND Employee.salary = 10000. (This is the JOIN for "Grade A".) It finds 2 matches - one for Nikhil and one for Akhil. So you get 2 rows from this JOIN.
2. It looks for any records where Salary.Salary = 20000 AND Employee.salary = 20000. (This is the JOIN for "Grade B".) It finds 1 match (assuming that Abhisek's "real" value is 20000, and not 2000.) So you get 1 row from this JOIN. You now have 3 rows.
3. It looks for any records where Salary.Salary = 10000 AND Employee.salary = 10000. (This is the JOIN for "Grade C".) It finds 2 matches - one for Nikhil and one for Akhil. So you get 2 rows from this JOIN. You now have 5 rows.
4. Finally, you end up with 1 more row for Shubham. Shubham didn't match anything in the JOIN, but it was part of the original SELECT ... FROM clause. So you still get that row returned. You now have 6 rows.

Finally, it was suggested by another respondent that you can eliminate the NULL result by doing this: WHERE grade <> NULL. That's wrong. That will eliminate all the rows. If you want to filter out the NULL values, you would need to add this: WHERE grade IS NOT NULL.

MySQL not null in left join condition

This is because "posts" does not contain the null-values and hence they can´t be filtered at that stage. The Null-values are only generated trough the join, when the server can´t find a corresponding row on the right table. So just put the not null in the where clause and it will work:

SELECT * FROM users U LEFT JOIN posts P ON P.userId = U.id WHERE userId IS NOT NULL;

(EDIT: You should use an inner join for productive work though, as it is the proper way and will give you much greater performance.)

You can also see all users who don´t have posts by inverting that:

SELECT * FROM users U LEFT JOIN posts P ON P.userId = U.id WHERE userId IS  NULL;

MySQL - why LEFT JOIN and NULL condition on NON-NULL column are returning all the rows

The condition on p1.category_id = p2.category_id and p1.price < p2.price and p2.price is null is always false. Therefore the resulting left join is each row from products (p1) filled up with NULL values for the products (p2) columns, e.g.

1, 'LG P880 4X HD', 336, 3, NULL, NULL, NULL, NULL

2, 'Google Nexus 4', 299, 2, NULL, NULL, NULL, NULL

...

12, 'Abercrombie Lake Arnold Shirt', 60, 1, NULL, NULL, NULL, NULL

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