How to use multiple WITH statements in one PostgreSQL query?

Getting your syntax right and boosting performance

When incorporating multiple CTEs, adherence to the correct syntax is of paramount importance:

• Assign a comma at the end of each CTE definition, except for the one immediately preceding the central query.
• Dodging syntax disasters in the WHERE clause is as simple as a meticulous inspection of temporary table conditions.
• When it comes to structuring JOINs, opting for INNER JOIN over IN or EXISTS promises more efficient table interactions.
• The EXISTS condition has an edge over IN when it comes to handling NULL values appropriately and providing a potential performance upswing.
• An INNER JOIN can trounce IN or EXISTS by leveraging table indexes more efficaciously.

Let's picture a smart way to chain CTEs:

WITH 
cte1 AS (SELECT * FROM table1 WHERE condition1),  -- Step 1: Filtering table1
cte2 AS (SELECT * FROM table2 WHERE condition2),  -- Step 2: Filtering table2
cte3 AS (SELECT * FROM cte1 JOIN cte2 ON cte1.key = cte2.key)  -- Step 3: Join our filtered heroes
SELECT * FROM cte3;  -- Step 4: Profit!


In the above instance, cte3 relies on the preceding CTEs, highlighting the interdependency aspect of multiple CTEs.

Mastering complex queries via CTEs

CTEs can help dissect otherwise intricate queries into digestible chunks:

• Apply CTEs to pre-filter the data, easing the strain on subsequent JOIN operations.
• Enable sequential chaining of CTEs to execute progressive transformations.
• Let temporary tables serve as isolated scopes defining intermediate findings to enable multi-leveled logic.

Skirting common pitfalls with NULLs and JOINS

Bypass potential pitfalls when integrating JOINs and dealing with NULLs:

• Using IN, and a NULL column can lead to an unexpectedly empty result set as NULL != NULL.
• EXISTS provides greater robustness, bypassing direct value comparison while accommodating NULLs gracefully.
• Be wary of JOIN conditions that could inadvertently omit rows having NULL values, affecting the accuracy of the results.

Supercharging your query via proper indexing

Optimal indexing is key to enhancing query performance, particularly when dealing with JOINs:

• Ensure indexes are aptly assigned to each table in the CTE, with special attention given to columns critical for JOINS.
• Analyse execution plans to customize indexes per the specific database workload.