如何比较不同表中的 2 列

Question

I'm trying to have a clausule where what compare a column from table A with a column from table B but the result dont bring nothing. Am I doing something wrong?

My query>

SELECT EMP, COD, VEV, SEQ, TIP, NUM, EMI, VEN, DTR, GRU, PRO, QTD, PVE, TOT, FAT, COM, COO, REF, VAR, hcli.NOM as "VENDEDOR", hemp.enc
 FROM hcov 
    INNER JOIN hcli ON hcov.COD = hcli.COD
    INNER JOIN hemp ON hemp.cod = hcov.emp 
WHERE (hcli.NOM NOT LIKE '%COOPERATIVA%')
AND (DTR >= date '2020-01-01')
AND (COD <> 24545 OR COD <> 10368 OR COD <> 13149 OR COD <> 10448 OR COD <> 11041 OR COD <> 30610 OR COD <> 6834)
AND (GRU <> 266 OR GRU <> 269 OR GRU <> 272)
AND ( (VEV <> 37125) AND (COO <> 987209 OR COO <> 23631 OR COO <> 927500 OR COO <> 22763 OR COO <> 38736 ) )
AND (hemp.enc <> hcov.cod) 

AND (hemp.enc <> hcov.cod) -> aparently this line is not working as should

The result is totally blank but both column is different.. so it should bring

Answer 1

The issue is because you are comparing values where (at least one of them) is NULL.

NULL is not considered a value - it is considered 'unknown'. It could be the value you're checking against, or it may not. Therefore, checking if anything = NULL results in a NULL answer - the answer is also unknown.

Note that even checking if NULL = NULL returns NULL. It may be that the first value is 1 and the second 1 too, therefore it would be true, Or it could be that the second value is 2. therefore it's false, As such, checking if NULL = NULL results in NULL.

You can check if something IS NULL - what this is essentially asking 'whether the value is unknown'. eg,

• if @a is NULL
• then checking IF @a = NULL results in NULL
• while checking IF @a IS NULL results in true

In other words, you need to work out how you want the process to work if hemp.enc is NULL and/or hcov.cod is NULL.

For example, you may change the line in the WHERE clause to

AND ((hemp.enc <> hcov.cod) 
     OR (hemp.enc IS NULL AND hcov.cod IS NOT NULL)
     OR (hemp.enc IS NOT NULL AND hcov.cod IS NULL)
    )

Here is a db<>fiddle with some examples of results of checking vs NULLs. Note that only the explicit IS NULL and IS NOT NULL actually return results.