Filter Table Before Applying Left Join

sql

subquery

ctes

index-optimization

byNikita Barsukov·Dec 20, 2024

To pre-filter data effectively during a LEFT JOIN, use a subquery. It filters a subset of the data to enhance performance. Here’s an efficient SQL pattern:

SELECT mt.id, mt.value, ft.data
FROM main_table mt
LEFT JOIN (
    SELECT id, data FROM sub_table WHERE condition
    -- Filter data like a pro. SQLight Saving Mode: Activated!
) AS ft ON mt.id = ft.id;

This ensures that main_table (mt) only links to those records of sub_table (ft) that satisfy the condition.

Patterns for high-performance queries

When it comes to maximum query performance, the following techniques earn your query stripes for speed plus complete inclusion of your customer data-records.

Subquery for pre-filtering before the join

Subquery saves computation because it allows filtering entry_table before the actual joining happens. It's like doing your laundry before packing for a trip. Less baggage, more fun!

Leverage CTEs to organize your work

CTEs or Common Table Expressions provide a temporary result set to your SQL statement. They improve clarity and maintainability, especially when your queries become as complex as a Rubik's cube.

Employ nested CTEs for more demanding filtering

Step up your game with nested CTEs for those multi-layered filtering challenges. Think of it like a multi-stage rocket blasting off to the moon! 🚀

Advanced pre-join filter optimization

Before we hit "Run" on our query, let's strap in for some advanced ways to maximize the bang for our buck at the pre-join filtering stage.

Mastery of the ON clause

The ON clause is hosting the grand party where everyone's invited. Proper usage ensures acustomer table party with no one left behind.

Emphasis on index optimization

To speed up the racehorse that is your query, use indexes strategically with WHERE clause of the sub-table. Turbocharge your race with an efficient pit crew (indexes) to cut down on unnecessary trips around the track (rows).

Remember to confirm your results

Upon proving your query skills in execution, pause for a result validation. It's like releasing an arrow after careful aim - and saving some for a follow-up if needed.

explain-codes / Sql / Filter Table Before Applying Left Join

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