Create an inline SQL table on the fly (for an excluding left join)

sql

prompt-engineering

best-practices

join

byNikita Barsukov·Nov 25, 2024

Here's a simple solution for excluding records using a left join and an inline SQL table. In this, I utilize the power of CTE (Common Table Expressions) and efficient VALUES clause:

-- CTE to the rescue, to filter unwanted Ids
WITH ExcludeThese AS ( 
    SELECT * FROM (VALUES (1), (2), (3)) AS TempTable(Id) -- Creating an Inline Filter
)
SELECT m.*
FROM MainTable m
LEFT JOIN ExcludeThese ON m.Id = ExcludeThese.Id -- Joining the Main table
WHERE ExcludeThese.Id IS NULL; -- Excluding Filtered Ids

This nifty SQL snippet sidesteps rows from MainTable having Id values of 1, 2, or 3 by creating and using a transitory set defined inline.

Unpacking the magic: SQL Server table value constructor

What's happening behind the scenes? We employ VALUES clause, a powerful facet of the SQL Server table value constructor (SQL Server 2008 onwards). This enables us to design an inline table by inserting multiple rows in a single statement.

When to use temporary tables?

When your use case needs ephemeral Ids that should be excluded from the prime dataset, you'll find delight in using this approach. Here, structuring inline tables using CTEs or subqueries is much neater and efficient than juggling with stored procedures or physical temporary tables.

Decoding SQL joins in daily life

Picture that you're at a train station overseeing two distinct tracks:

Track A (🛤️): [Train 1, Train 2, Train 3]
Track B (🛤️): [Train 2, Train 3, Train 4]

Your mission is to spot which trains from Track A are missing from Track B.

🛤️A *(Left Join)* 🛤️B: [Train 1]

Voila! Using an Excluding Left Join is akin to flagging down those special trains from Track A that are absent from Track B track.

🚩 [Train 1] - our unique train not seen on both tracks.

Imagine how our inline table - ExcludeThese, acts as that special filter enabling the SQL engine to flag and exclude unique Ids (Trains here 🚂) from the MainTable.

Strategies for SQL efficiency

The real art behind writing performant SQL queries lies in balance and strategy. Here are some:

Use of primary keys and indexes

If you find yourself working with actual temporary tables, remember to create them with PRIMARY KEYs and suitable indexes for speeding up comparison operations.

Structured clarity

An ordered inline table reflecting the potential order of your main table does the dual job - it helps the SQL engine as well as the SQL script reader understand the temporary data set better.

Duplication doom

When merging multiple result sets using UNION ALL, remember, it doesn't eliminate duplicates. To ensure all IDs are unique, consider using the de-duping prowess of UNION or make sure your VALUES data is already distinct.

Clean-up macro

In scenarios where you're engineering temporary tables, and not merely CTEs remember to clean up after the operations with a DROP TABLE command.

Test and demonstrate

Feel free to dip your toes in and brew your SQL snippets on SQLFiddle. This lets the crowd run your SQL snippet and see the actual outcome - all in an isolated, controlled environment.

explain-codes / Sql / Create an inline SQL table on the fly (for an excluding left join)

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