Join two SQL queries [duplicated]

You do not see clearly what you are trying because initially you define queries that change in the unified query, but I understand that it will be something similar to this. I have also changed the ORs for AND since I understand that you want all the conditions to be met. I hope that from this query you can adapt it to what you need.

SELECT DISTINCT CodContrato, FechaBaja, FechaFin, FechaFinProrroga
FROM Contrato INER JOIN DatoAuxiliarContrato ON Contrato.CodContrato=DatoAuxiliarContrato.CodContrato
WHERE FechaBaja != FechaFin AND FechaBaja != FechaFinProrroga AND FechaBaja BETWEEN '20180101 00:00:00' AND '20181231 23:59:59'

I do not know if the dates that have the same name in both tables should match (I suppose so), but if they are different (once you understand the example and can adapt it) the query would be something like this:

SELECT DISTINCT Contrato.CodContrato, Contrato.FechaBaja AS ContratoFechaBaja, Contrato.FechaFin AS ContratoFechaFin, DatoAuxiliarContrato.FechaBaja AS DatoAuxiliarContratoFechaBaja, DatoAuxiliarContrato.FechaFin AS DatoAuxiliarContratoFechaFin, DatoAuxiliarContrato.FechaFinProrroga
FROM Contrato INNER JOIN DatoAuxiliarContrato ON Contrato.CodContrato=DatoAuxiliarContrato.CodContrato 
WHERE (Contrato.FechaBaja != Contrato.FechaFin) AND (Contrato.FechaBaja BETWEEN '20180101 00:00:00' AND '20181231 23:59:59')) 
AND ((DatoAuxiliarContrato.FechaFinProrroga != '') AND (DatoAuxiliarContrato.FechaFinProrroga BETWEEN '20180101 00:00:00' AND '20181231 23:59:59'))

I advise you to keep an eye on the different types of JOINS available and use the one that best suits your problem: Types of JOINS

Using a subselect

SELECT DISTINCT c.CodContrato, c.FechaBaja, c.FechaFin
FROM Contrato c
WHERE (c.FechaBaja != c.FechaFin OR c.FechaBaja 
NOT IN (SELECT DISTINCT FechaFinProrroga 
FROM DatoAuxiliarContrato WHERE CodContrato=c.CodContrato AND (FechaFinProrroga != '') AND (FechaFinProrroga BETWEEN '20180101 00:00:00' AND '20181231 23:59:59')) )
AND (c.FechaBaja BETWEEN '20180101 00:00:00' AND '20181231 23:59:59')

You tried doing:

SELECT DISTINCT
    con.CodContrato,
    con.FechaBaja,
    con.FechaFin,
    dac.FechaFinProrroga
FROM
    Contrato AS con,
    DatoAuxiliarContrato AS dac
WHERE
    (
        con.FechaBaja != con.FechaFin 
        OR con.FechaBaja != dac.FechaFinProrroga
    )
    AND con.FechaBaja BETWEEN '20180101 00:00:00' AND '20181231 23:59:59';

ANSI SQL has a feature called EQUI JOIN . Viewing the query you wanted to make, just modify it to fit the standard.

I hope it serves you.