What am I missing in my SQL script?

Question

I'm very new to SQL, and I am struggling to see what I've missed in this script.

Each time I try to run it, it takes ages and ends up with an error due to insufficient memory.

select r.CONSTITUENT_ID
     , case when r.SEX = '1'then 'Male'
           when r.SEX = '2' then 'Female' else 'Unknown'end as Gender
     , case when r.KEY_INDICATOR = 'O' then 'Organisation'
            when r.KEY_INDICATOR = 'I' then 'Individual'end as record_Classification
     , case when cc.CODE = '1924' then 'Bristol Affiliated'
            when cc.CODE = '1963' then 'Gloucester Affiliated'
            when cc.CODE = '2045' then 'Wiltshire Affiliated'
            else 'n/a' end as DW_Affiliation
from RECORDS r, CONSTITUENT_CODES cc

Answer 1

This

from RECORDS r,CONSTITUENT_CODES cc` 

is cross join what means show me every row from RECORDS with every row CONSTITUENT_CODES so it returns [rows number from RECORDS]*[rows number from CONSTITUENT_CODES ] rows. You have to use join or where to eliminate cross join

from RECORDS r,CONSTITUENT_CODES cc
where r.column  = cc.column

or

from RECORDS r
join CONSTITUENT_CODES cc on r.column  = cc.column

I don't know your table structure but maybe this will work

select r.CONSTITUENT_ID
     , case when r.SEX = '1'then 'Male'
           when r.SEX = '2' then 'Female' else 'Unknown'end as Gender
     , case when r.KEY_INDICATOR = 'O' then 'Organisation'
            when r.KEY_INDICATOR = 'I' then 'Individual'end as record_Classification
     , case when cc.CODE = '1924' then 'Bristol Affiliated'
            when cc.CODE = '1963' then 'Gloucester Affiliated'
            when cc.CODE = '2045' then 'Wiltshire Affiliated'
            else 'n/a' end as DW_Affiliation
from RECORDS r, CONSTITUENT_CODES cc
where r.CONSTITUENT_ID = cc.CONSTITUENT_ID 

with recommended join style it looks like

select r.CONSTITUENT_ID
     , case when r.SEX = '1'then 'Male'
           when r.SEX = '2' then 'Female' else 'Unknown'end as Gender
     , case when r.KEY_INDICATOR = 'O' then 'Organisation'
            when r.KEY_INDICATOR = 'I' then 'Individual'end as record_Classification
     , case when cc.CODE = '1924' then 'Bristol Affiliated'
            when cc.CODE = '1963' then 'Gloucester Affiliated'
            when cc.CODE = '2045' then 'Wiltshire Affiliated'
            else 'n/a' end as DW_Affiliation
from RECORDS r 
join CONSTITUENT_CODES cc on r.CONSTITUENT_ID = cc.CONSTITUENT_ID 

I see in RECORDS is CONSTITUENT_ID this column can be use to join with column id from CONSTITUENT_CODES - I guess CONSTITUENT_ID