How to Filter Data in PostgreSQL

How do I filter data in PostgreSQL?

Use the WHERE clause with logical, comparison, and pattern operators to return only the rows that match specified conditions.

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Description

Why use the WHERE clause to filter data?

Filtering keeps result sets small, improves performance, and returns only business-relevant records.In reporting dashboards or APIs, sending fewer rows lowers latency and network costs.

Which operators can I combine in WHERE?

Combine comparison (=, <>, >, <, >=, <=), range (BETWEEN), set membership (IN), pattern matching (LIKE, ILIKE, SIMILAR TO), full-text search (@@), and JSON operators (->, ->>).

How do I filter on multiple conditions?

Chain expressions with AND, OR, and NOT. PostgreSQL evaluates conditions left to right, honoring parentheses for precedence.

Can I filter using subqueries?

Yes.Use WHERE column IN (SELECT …) for membership or WHERE EXISTS (SELECT 1 …) for existence checks.

When should I add an index for faster filtering?

Add B-tree indexes on columns frequently used in WHERE, especially in highly selective filters.Use partial indexes to include only commonly filtered subsets.

Example: How to find high-value orders this month?

SELECT o.id, o.total_amount FROM Orders o WHERE o.order_date >= date_trunc('month', CURRENT_DATE) AND o.total_amount >= 500;

Best practice: Always qualify columns

Alias tables and prefix columns (e.g., o.order_date) to avoid ambiguous references and improve readability.

What mistakes should I avoid?

Don’t wrap numeric columns in functions in WHERE; it prevents index use.Also, be careful with NOT LIKE patterns—they can’t use a B-tree index.

How do I filter JSON data?

Use ->> to extract a text value, then compare. For JSONB containment, use the @> operator.

How do I filter rows case-insensitively?

Use ILIKE for pattern matching or lower(column) = lower(value) if an expression index on lower(column) exists.

Why How to Filter Data in PostgreSQL is important

How to Filter Data in PostgreSQL Example Usage


-- Customers who placed high-value orders in August 2024
SELECT  c.id, c.name, SUM(o.total_amount) AS august_spend
FROM    Customers   c
JOIN    Orders      o ON o.customer_id = c.id
WHERE   date_trunc('month', o.order_date) = DATE '2024-08-01'
  AND   o.total_amount > 200
GROUP BY c.id, c.name
ORDER BY august_spend DESC;

How to Filter Data in PostgreSQL Syntax


SELECT column_list
FROM   table_name
WHERE  condition1 [AND|OR] condition2 [...]

-- Common operators
-- Comparison: =, <>, >, <, >=, <=
-- Range: BETWEEN val1 AND val2
-- Membership: IN (val1, val2, ...)
-- Pattern: [NOT] LIKE pattern [ESCAPE escape_char]
-- Case-insensitive: ILIKE pattern
-- Null test: IS [NOT] NULL
-- Existence: EXISTS (subquery)
-- JSON containment: jsonb_column @> '{"key":"value"}'

-- Ecommerce example
SELECT c.id, c.name, c.email
FROM   Customers c
WHERE  c.created_at >= CURRENT_DATE - INTERVAL '30 days'
  AND  c.email ILIKE '%@example.com';

Common Mistakes

Wrapping columns in functions (e.g., WHERE upper(name) = 'JOHN') disables indexes. Fix by creating an expression index or comparing with ILIKE.
Using leading wildcards in LIKE ('%shoe') forces a sequential scan. Fix with trigram indexes or reverse the string if appropriate.