Question

I have to check a few foreign keys in a loop and need to replace the name of the foreign key field with a variable (@fk_field for example).

My query looks like this:

    SELECT 
        ad.id 
        , ad.@fk_field
        , a1.@jnktn_field
    FROM @tblname tbl LEFT JOIN @jnktn_tbl jnktn ON tbl.@fk_field=jnktn.@jnktn_field
    WHERE ad.FK1 <> isnull(a1.ID,0)

Apparently it’s not possible to use a variable directly after a point like I did, but I need to do this to refer to the right table. So, how do I use variables to insert the fieldnames which were set before the SQL query?

Answer 1

Depending on where and how you use that query you could use

SQLCMD variables

:setvar FKFIELD field_name

SELECT 
    ad.id 
    , ad.@fk_field
    , a1.@jnktn_field
FROM @tblname tbl LEFT JOIN @jnktn_tbl jnktn ON tbl.$(FKFIELD)=jnktn.field
WHERE ad.FK1 <> isnull(a1.ID,0)

EXECUTE Statement

EXEC('SELECT 
    ad.id 
    , ad.@fk_field
    , a1.@jnktn_field
FROM @tblname tbl LEFT JOIN @jnktn_tbl jnktn ON tbl.' + @fk_field +'=jnktn.field
WHERE ad.FK1 <> isnull(a1.ID,0)')

Multiple JOIN clauses

SELECT 
    ad.id 
    , ad.field
    , a1.field
FROM @tblname tbl 
LEFT JOIN @jnktn_tbl jnktn ON tbl.Field1=jnktn.field and @fk_field = 'fieldName1'
LEFT JOIN @jnktn_tbl jnktn ON tbl.Field2=jnktn.field and @fk_field = 'fieldName2'
LEFT JOIN @jnktn_tbl jnktn ON tbl.Field3=jnktn.field and @fk_field = 'fieldName3'
WHERE ad.FK1 <> isnull(a1.ID,0)

UPDATE: This last example should require a bit of effort to get the query right and might not work at all depending on your exact requirements. The idea is to join to all referenced tables and set all fields from these tables to NULL except for the table with the key specified in the variable.