How to Filter Data in PostgreSQL

Galaxy Glossary

How do I filter rows using the WHERE clause in PostgreSQL?

The WHERE clause filters table rows by evaluating a Boolean condition and returning only rows that match.

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Description

How to Filter Data in PostgreSQL

Use WHERE and FILTER clauses to return only the rows that meet your conditions, reduce scan time, and speed up queries.

What does the WHERE clause do?

The WHERE clause evaluates a Boolean expression for every row and returns only rows where the expression is true. This filters data at the row level before aggregation or ordering.

What is the syntax for filtering rows?

Start a SELECT, list the columns, add FROM table_name, then append WHERE followed by one or more conditions. Combine conditions with AND, OR, and parentheses for precedence.

Basic equality filter

Retrieve users from the USA:
SELECT * FROM users WHERE country = 'USA';

Filter with logical operators

Combine checks:
SELECT * FROM users WHERE active AND (country = 'USA' OR country = 'CAN');

Pattern matching with LIKE and ILIKE

Find names that start with S, case-insensitive:
SELECT * FROM users WHERE name ILIKE 's%';

Range filtering with BETWEEN

Return orders placed in 2024:
SELECT * FROM orders WHERE order_date BETWEEN '2024-01-01' AND '2024-12-31';

Null-safe filtering

Only rows where phone is missing:
SELECT * FROM customers WHERE phone IS NULL;

When should I use FILTER in aggregates?

Use FILTER (WHERE ...) inside aggregate functions to include only matching rows without a subquery.

Aggregate filter syntax

SELECT count(*) FILTER (WHERE status = 'open') AS open_count FROM tickets;

Best practices for filtering data

1. Create indexes on columns often used in WHERE.
2. Compare to constants or parameters, not functions, to keep indexes usable.
3. Use EXPLAIN to verify index usage.

Full example query

SELECT id, name, created_at FROM customers WHERE country = 'USA' AND active = true ORDER BY created_at DESC LIMIT 50;

How to avoid common mistakes

Use IS NULL instead of = NULL, and enclose OR conditions in parentheses when mixed with AND.

Why How to Filter Data in PostgreSQL is important

How to Filter Data in PostgreSQL Example Usage


SELECT id, name
FROM public.customers
WHERE country = 'USA' AND active = true
ORDER BY created_at DESC;

How to Filter Data in PostgreSQL Syntax


SELECT column_list
FROM table_name
WHERE condition1 [AND|OR condition2 ...]
[ORDER BY column_list]
[LIMIT n];

-- Aggregate example
SELECT agg_func(col) FILTER (WHERE condition) FROM table;

Common Mistakes

Comparing NULL with = NULL. PostgreSQL treats NULL as unknown, so = NULL returns NULL, not true. Fix by using IS NULL or IS NOT NULL.
Mixing AND and OR without parentheses. This can change logic unexpectedly because AND has higher precedence. Group OR conditions in parentheses to clarify intent.