Sql Left Join first match only

sql

join

subqueries

performance

byAnton Shumikhin·Nov 28, 2024

In order to achieve a first match only on a SQL LEFT JOIN, you can expertly utilize ROW_NUMBER() in combination with PARTITION BY. This pair works together to rank all entries in the joined dataset. Here's a simplified coding scenario:

SELECT A.*, B.*
FROM A
LEFT JOIN (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY join_key ORDER BY order_preference) AS rn
    FROM B
) AS B_first ON A.join_key = B_first.join_key AND B_first.rn = 1

In the above query, replace join_key with the column you are joining on and order_preference with the column that determines the first record. This trick narrows down B with only the first result for each corresponding record in A.

Comprehensive practices

Selecting diverse records

Occasionally, you'll encounter repeated values in your datasets. Use DISTINCT in your subqueries to weed out these annoying clones:

SELECT DISTINCT first_name, LAST_name, MIN(join_date) as initiation_date
FROM memberbase
GROUP BY first_name, last_name -- You thought you can duplicate yourself, huh?

The above snippet utilises the MIN() function to solidify uniqueness in your SQL query result.

Strategies to outstrip performance issues

Craft your code smartly by selecting only required columns and incorporating indexes to be the Fast and Furious in the SQL world.

Mastering complex cases

Deep relational data requires an extra touch. You can use correlated subqueries for this purpose. Here's an example:

SELECT A.ID, A.VALUE, B.SUB_FIELD
FROM A
LEFT JOIN B ON B.ID = (
    SELECT TOP 1 sub.id
    FROM B sub
    WHERE sub.A_ID = A.ID
    ORDER BY SOME_CRITERIA -- Order is the first step to sanity
)

String comparisons in uniqueness

For some queries, the devil lies in the case (upper or lower) of the strings you're comparing. Use functions like UPPER() or LOWER() to work your SQL magic:

SELECT *
FROM A
LEFT JOIN B ON UPPER(A.NAME) = UPPER(B.NAME) -- Because JOHN DOE is the same as john doe

Managing order and groups

The ORDER BY clause in SQL is like the director of an orchestra—it can drastically change the result. Here's a instance where we want most recent orders per customer:

SELECT *
FROM CUSTOMERS
LEFT JOIN (
    SELECT *, ROW_NUMBER() OVER (PARTITION BY CUSTOMER_ID ORDER BY ORDER_DATE DESC) AS RN
    FROM ORDERS
) AS RECENT_ORDER ON CUSTOMERS.CUSTOMER_ID = RECENT_ORDER.CUSTOMER_ID AND RECENT_ORDER.RN = 1 -- No we don't mean Star Wars RN1, sorry!