Writing a query for multiple fields using IN - sql

Question

SELECT a.First_Name,b.EMPID,c.Phn
FROM NAME a,Emp b,Phone c
WHERE b.Emptype = 'New'
AND a.First_name,b.Last_name,C.Phn = (SELECT a.First_Name,b.Last_name,c.Phn
FROM NAME a,Emp b,Phone c
WHERE b.Emptype = 'Old')
Basically, I want to search for new customers which have the same details (First name, Last name and Phone) as the old customers. An old customer can be converted into a new customer but its details are retained. Hence, the only thing that changes is the emptype.
Eg. (John McEnroe 47589876 Old) when converted becomes (John McEnroe 475898876 New)
[First_name,Last_name,Phone,Emptype]

Answer 1

Dude, here's a query that uses EXISTS.
The main difference between Exists and IN is that Exists queries using INDEX but IN goes like a lot of OR toghether.
SELECT a.First_Name
,b.EMPID
,c.Phn
FROM NAME a
,Emp b
,Phone c
WHERE b.Emptype = 'New'
AND EXISTS (Select TOP 1 1 FROM
NAME x
,Emp y
,Phone z
WHERE
x.First_Name = a.First_Name
AND y.Last_name = b.Last_name
AND z.Phn = c.Phn
AND y.Emptype = 'Old'
)
Now, looking at your query, you have 3 tables, not joined toghether (Name, Emp, Phone) so you have a real mess with the data that will come as an output. Will be, basically this formula:
(1st Table rows qty) Multiplied by (2nd Table rows qty) Multiplied by (3rd Table rows qty)
Means that if each table have 100 rows, you'll get this:
100 x 100 x 100 = 1000000 Rows in the result (Awful right?)

Answer 2

WITH tempTable as (
SELECT a.First_Name,b.EMPID,c.Phn
FROM NAME a,Emp b,Phone c
where b.Emptype = 'Old'
)
select a.First_Name,b.EMPID,c.Phn
FROM NAME a,Emp b,Phone c,tempTable t
where b.Emptype = 'Old' and a.First_Name = t.First_Name
AND b.Last_name = t.Last_name
AND c.Phn = t.Phn;
try this... or use subquery...

Answer 3

You can use multiple columns for an IN query:
AND (a.First_name,b.Last_name,C.Phn) IN (SELECT a.First_Name,b.Last_name,c.Phn
FROM ... )