How to select rows with no matching entry in another table?

I would use EXISTS expression since it is more powerful, you can e.g. more precisely choose rows you would like to join. In the case of LEFT JOIN, you have to take everything that's in the joined table. Its efficiency is probably the same as in the case of LEFT JOIN with null constraint.

SELECT t1.ID
FROM Table1 t1
WHERE NOT EXISTS (SELECT t2.ID FROM Table2 t2 WHERE t1.ID = t2.ID)

SELECT id FROM table1 WHERE foreign_key_id_column NOT IN (SELECT id FROM table2)

Table 1 has a column that you want to add the foreign key constraint to, but the values in the foreign_key_id_column don't all match up with an id in table 2.

1. The initial select lists the ids from table1. These will be the rows we want to delete.
2. The NOT IN clause in the where statement limits the query to only rows where the value in the foreign_key_id_column is not in the list of table 2 ids.
3. The SELECT statement in parenthesis will get a list of all the ids that are in table 2.

Here's a simple query:

SELECT t1.ID
FROM Table1 t1
    LEFT JOIN Table2 t2 ON t1.ID = t2.ID
WHERE t2.ID IS NULL

The key points are:

1. LEFT JOIN is used; this will return ALL rows from Table1, regardless of whether or not there is a matching row in Table2.

2. The WHERE t2.ID IS NULL clause; this will restrict the results returned to only those rows where the ID returned from Table2 is null - in other words there is NO record in Table2 for that particular ID from Table1. Table2.ID will be returned as NULL for all records from Table1 where the ID is not matched in Table2.