MySQL Select entries missing from another table

Question

I'm attempting to get a list of values that don't exist in another table for a specific date.

Table Layouts:

table1 (TABLE)
 id INT(10)

table2 (TABLE)
 id INT(10)
 table1id INT(10)
 dateupdated DATETIME(19)

SQLFiddle

Query that I thought would work that isn't.

SELECT t1.id FROM Table1 as t1
    JOIN Table2 as t2
    ON t1.id=t2.table1id
    WHERE date(t2.dateupdated) != DATE(NOW())

I'm expecting id 7 and 8 to come back from the above query, since they don't exist in Table2. But I'm getting no rows back.

Reversing the Query to match = DATE(NOW()) gives me the lines that do have matches

SELECT t1.id FROM Table1 as t1
JOIN Table2 as t2
ON t1.id=t2.table1id
WHERE date(t2.dateupdated) = DATE(NOW())

Response:

id
1
2
3
4
5
6
9
10

Any guidance would be appreciated, I'm still learning MySQL. But I'm missing something very simple here.

Answer 1

http://sqlfiddle.com/#!9/7a9fc/11

SELECT t1.id FROM Table1 as t1
    LEFT JOIN Table2 as t2
    ON t1.id=t2.table1id
     AND date(t2.dateupdated) = DATE(NOW())
    WHERE t2.id IS NULL;

Answer 2

Since there is always more than one way, another way is using the NOT IN operator with a subquery (since MySQL doesn't support MINUS).

SELECT t1.id FROM Table1 as t1
WHERE t1.id NOT IN
(SELECT t2.table1id FROM Table2 as t2
    WHERE date(t2.dateupdated) = DATE(NOW()))

For DBMS that do support MINUS, you could do:

SELECT t1.id FROM Table1 as t1
MINUS
SELECT t2.table1id FROM Table2 as t2
    WHERE date(t2.dateupdated) = DATE(NOW())

Answer 3

This is the way that I prefer to do it:

SELECT id FROM Table1
    WHERE id not in (select table1id from Table2 where date(dateupdated) = date(NOW()))

Answer 4

Learning mysql I found this invaluable reference: http://blog.codinghorror.com/a-visual-explanation-of-sql-joins/

Hope you find it useful!

Since the JOIN doesn't retrieve NULL rows, you have to use a LEFT or RIGHT JOIN to deal with empty ROWS.