How to CAST Data Types in PostgreSQL

How do I cast data types in PostgreSQL like in Oracle?

CAST converts a value from one data type to another.

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Description

What does CAST do in PostgreSQL?

CAST changes a value’s data type, letting you treat, store, or compare the value as another compatible type. It is ANSI-SQL compliant and mirrors Oracle’s CAST function, so migration is straightforward.

What is the basic CAST syntax?

Use either the keyword CAST(value AS target_type) or the shorthand value::target_type. Both perform the same conversion.

How do I cast strings to numbers?

Convert customer IDs stored as text to integers: SELECT CAST(id AS integer) FROM Customers;.The query returns integer IDs that can be summed or compared numerically.

How do I cast timestamps to dates?

Strip time parts for daily sales: SELECT order_date::date, SUM(total_amount) FROM Orders GROUP BY order_date::date;. The cast enables GROUP BY on pure dates.

How do I cast between numeric types safely?

Down-casting can lose precision. Example: SELECT CAST(price AS numeric(10,2)) FROM Products;. Always pick a scale and precision large enough to avoid rounding errors.

Can I cast NULLs?

Yes.SELECT CAST(NULL AS varchar); makes the planner treat the NULL as varchar for union or comparison operations.

When should I prefer the :: operator?

The :: operator is shorter and favored in PostgreSQL-specific code. Use CAST() when you need Oracle-compatible SQL or want clearer ANSI syntax.

How to cast JSON fields?

Extract and cast: SELECT (details->>'quantity')::integer AS qty FROM OrderItems;.The text extracted from JSON is cast to integer for arithmetic.

Best practices for using CAST

1) Validate input formats before casting strings.
2) Use domain-specific types (e.g., money) where appropriate.
3) Index the casted expression if querying often.

Common mistakes to avoid

1) Casting incompatible types: CAST('abc' AS integer) throws an error. Use regex or try_cast-like PL/pgSQL wrappers to pre-validate.
2) Precision loss: casting 19.999 to numeric(5,2) rounds to 20.00.Pick correct precision.

Need Oracle-style CAST during migration?

Replace Oracle’s CAST with identical PostgreSQL syntax. Most expressions work unchanged. Only date format masks differ; use TO_DATE or TO_TIMESTAMP first, then CAST if needed.

Why How to CAST Data Types in PostgreSQL is important

How to CAST Data Types in PostgreSQL Example Usage


-- Show orders where the stored text customer_id equals 42 after casting
SELECT id, customer_id::integer, total_amount
FROM Orders
WHERE customer_id::integer = 42;

How to CAST Data Types in PostgreSQL Syntax


CAST ( expression AS target_type )

-- Shorthand
expression::target_type

-- Examples in ecommerce context
-- 1. String to integer
SELECT CAST(customer_id AS integer) FROM Orders;
-- 2. Numeric to text
SELECT total_amount::text FROM Orders;
-- 3. Timestamp to date for grouping
SELECT order_date::date, COUNT(*) FROM Orders GROUP BY order_date::date;

Common Mistakes

Casting incompatible literals e.g., CAST('abc' AS integer) causes ERROR: invalid input syntax. Fix by validating with regex or using NULLIF/CASE before casting.
Ignoring precision when casting numerics. Price 19.999 cast to numeric(5,2) becomes 20.00. Fix by choosing adequate precision or rounding intentionally with ROUND().