Question

Lets say I have 2 tables T1 and T2. I need all values from table T1 that does not exist in table T2. How would that SQL look like?

T1:         T2:
T1.ID       T2.ID   
T1.Value    T2.T1ID 
T1.Date     T2.Value

Answer 1

I need all values from table T1 that does not exist in table T2. How would that SQL look like?

You mean you need every T1.Value that does not match any T2.Value?

If so, you can use something like this:

SELECT * FROM T1
WHERE Value NOT IN (SELECT Value FROM T2)

MySQL, however, may not be very good at finding optimal plan for executing this query, so if your measurements show a performance problem, try something like this:

SELECT DISTINCT T1.*
FROM T1 LEFT JOIN T2 ON T1.Value = T2.Value
WHERE T2.ID IS NULL

Note the DISTINCT clause, which ensures T1 rows are not “multiplied” by the JOIN.

Or even like this, if you only care about T1.Value:

SELECT DISTINCT T1.Value
...