IF / Case statment in SQL - sql

Question

I have a column where I have 0 or 1. I like to do the following set up:
If 0 than put / use the Region_table (here I have regions like EMEA, AP,LA with finished goods only) and when it 1 then put / use the Plant_table (here I have plants with non-finished goods) data's.
I tried to write it in 2 different statements but it is not good:
,Case
when [FG_NFG_Selektion] = '0' Then 'AC_region'
End as 'AC_region'
,Case
when [FG_NFG_Selektion] = '1' Then 'AC_plant'
End as 'AC_plant'

Answer 1

I'm not 100% clear on what you're looking for, but if you want to get data from different tables based on the value in the [FG_NFG_Selektion] field, you can do something like this:
SELECT
CASE
WHEN [FG_NFG_Selektion] = '0' THEN r.some_col -- If 0, use value from "region" table
WHEN [FG_NFG_Selektion] = '1' THEN p.some_col -- If 1, use value from "plant" table
END AS new_field
FROM MyTable t
LEFT JOIN AC_region r ON t.pk_col = r.pk_col -- get data from "AC_region" table
LEFT JOIN AC_plant p ON t.pk_col = p.pk_col -- get data from "AC_plant" table
;
If [FG_NFG_Selektion] is a numeric field, then you should remove the single quotes: [FG_NFG_Selektion] = 0.

Answer 2

I would strongly recommend putting the conditions in the ON clauses:
SELECT COALESCE(r.some_col, p.some_col) as som_col
FROM t LEFT JOIN
AC_region r
ON t.pk_col = r.pk_col AND
t.FG_NFG_Selektion = '0' LEFT JOIN
AC_plant p
ON t.pk_col = p.pk_col AND
t.FG_NFG_Selektion = '1';
Why do I recommend this? First, this works correctly if there are multiple matches in either table. That is probably not an issue in this case, but it could be in others. You don't want to figure out where extra rows come from.
Second, putting the conditions in the ON clause allows the optimizer/execution engine to take advantage of them. For instance, it is more likely to use FG_NFG_Selektion in an index.