MySQL select rows where left join is null

Question

I have these MySQL tables:

table1:

id | writer
1  | Bob   
2  | Marley
3  | Michael

table2:

user_one | user_two
   1     | 2

And this query:

SELECT table1.id FROM table1 LEFT JOIN table2 ON table1.id = table2.user_one

This query will return all rows of table1 which are 1,2,3

I want to select only rows which are not found in the left joint. So it should return only row with id 3

I want sort of the opposite of INNER JOIN which will select only the rows which are found in the join. How to get the opposite like if left join exists, ignore it and move to the next row. Hope i'm clear

Solution 1

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;

Solution 2

One of the best approach if you do not want to return any columns from table2 is to use the NOT EXISTS

SELECT  table1.id 
FROM    table1 T1
WHERE
  NOT EXISTS (SELECT *
              FROM table2 T2
              WHERE T1.id = T2.user_one
                  OR T1.id = T2.user_two)

Semantically this says what you want to query: Select every row where there is no matching record in the second table.

MySQL is optimized for EXISTS: It returns as soon as it finds the first matching record.

One more note to this: When you check against a nullable column in the joined table, depending on your needs, you may have to use NOT EXISTS (or check against other columns when using LEFT JOIN), because MySQL won't be able to distinguish between a column which is NULL, but there is an existing record in the joined table and a column which is generated as NULL because the joined table have no matching record.

Solution 3

Here is a query that returns only the rows where no correspondance has been found in both columns user_one and user_two of table2:

SELECT T1.*
FROM table1 T1
LEFT OUTER JOIN table2 T2A ON T2A.user_one = T1.id
LEFT OUTER JOIN table2 T2B ON T2B.user_two = T1.id
WHERE T2A.user_one IS NULL
    AND T2B.user_two IS NULL

There is one jointure for each column (user_one and user_two) and the query only returns rows that have no matching jointure.

Hope this will help you.

Solution 4

SELECT table1.id 
FROM table1 
LEFT JOIN table2 ON table1.id = table2.user_one
WHERE table2.user_one is NULL

Solution 5

Try following query:-

 SELECT table1.id 
 FROM table1 
 where table1.id 
 NOT IN (SELECT user_one
         FROM Table2
             UNION
         SELECT user_two
         FROM Table2)

Hope this helps you.

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