VLookup in SQL? - Joining to only pick out the top row - sql

Question

I am trying to get just the first row from a JOIN in SQL. Something similiar to Vlookup in Excel.
I have the following tables
CREATE TABLE customer_lookup (
customer_product varchar(50),
supplier_product varchar(50),
customer_code varchar(10)
)
CREATE TABLE supplier (
part_number varchar(50)
)
INSERT INTO customer_lookup (
customer_product,
supplier_product,
customer_code ) VALUES ('CONTAINER', 'BOX', 'CUST01')
INSERT INTO customer_lookup (
customer_product,
supplier_product,
customer_code ) VALUES ('CONTAINER', 'BOX', 'CUST02')
INSERT INTO customer_lookup (
customer_product,
supplier_product,
customer_code ) VALUES ('FABRIC', 'MATERIAL', 'CUST01')
INSERT INTO supplier ( part_number ) VALUES ('FABRIC')
INSERT INTO supplier ( part_number ) VALUES ('CONTAINER')
INSERT INTO supplier ( part_number ) VALUES ('PAINT')
and my query is
SELECT
s.part_number, c.supplier_product, c.customer_code
FROM
supplier s
LEFT JOIN
(
SELECT * FROM customer_lookup t
) c
ON s.part_number = c.customer_product
http://sqlfiddle.com/#!6/716b5/1
The result I am trying to get is
part_number supplier_product customer_code
FABRIC MATERIAL CUST01
CONTAINER BOX CUST01
PAINT (null) (null)
but the above SQL query produces
part_number supplier_product customer_code
FABRIC MATERIAL CUST01
CONTAINER BOX CUST01
CONTAINER BOX CUST02
PAINT (null) (null)
I don't care that the row with CONTAINER is missing customer_code CUST02. I just need to top one
I have tried
SELECT
s.part_number, c.supplier_product, c.customer_code
FROM
supplier s
LEFT JOIN
(
SELECT TOP 1 * FROM customer_lookup t
) c
ON s.part_number = c.customer_product
but this just nulls out both FABRIC and PAINT rows
Any help would be appreciated

Answer 1

You can use GROUP BY and MAX to achieve what you're looking for
SELECT
s.part_number, c.supplier_product, MAX(c.customer_code)
FROM
supplier s
LEFT JOIN
(
SELECT * FROM customer_lookup t
) c
ON s.part_number = c.customer_product
GROUP BY s.part_number, c.supplier_product
For every part_number and supplier_product unique identifying combination, you want the highest customer_code value.

Answer 2

If you don't care which row qualifies as the top row, as long as it returns one row at most, then you can use the row_number window function with order by null.
SELECT s.part_number, c.supplier_product, c.customer_code
FROM supplier s
LEFT JOIN (SELECT *,
row_number() over (partition by customer_product order by null) as rn
FROM customer_lookup) c
ON s.part_number = c.customer_product
AND c.rn = 1
If you do care which row gets picked, then just modify the order by clause accordingly.

Answer 3

You can simply use CROSS APPLY to get your results, the main benefit here is that you are not using aggregation (GROUP BY)
SELECT
s.part_number, c.supplier_product, c.customer_code
FROM
supplier s
CROSS APPLY
(
SELECT TOP 1 * FROM customer_lookup t
WHERE s.part_number = t.customer_product
ORDER BY t.customer_code
) c
You should also add an ORDER BY to ensure the results are order the way you want them to be (I have added this in for you).
You should also define columns that you are using rather than using an asterisk (*) but that's up to you (I've left this as is for now)
http://sqlfiddle.com/#!6/716b5/17

Answer 4

If you're wanting the results you showed and in the order you showed them
SELECT
s.part_number, c.supplier_product, MIN(c.customer_code)
FROM
supplier s
LEFT JOIN
(
SELECT * FROM customer_lookup t
) c
ON s.part_number = c.customer_product
GROUP BY s.part_number, c.supplier_product
ORDER BY c.supplier_product DESC