Casting refers to the operation of converting a value in a particular data type to the corresponding value in another data type. Casting can occur either implicitly or explicitly. The syntax described here performs an explicit cast. More information on casting can be found on the typecasting page.

Explicit Casting

The standard SQL syntax for explicit casting is CAST(expr AS TYPENAME), where TYPENAME is a name (or alias) of one of DuckDB's data types. DuckDB also supports the shorthand expr::TYPENAME, which is also present in PostgreSQL.

SELECT CAST(i AS VARCHAR) AS i
FROM generate_series(1, 3) tbl(i);

SELECT i::DOUBLE AS i
FROM generate_series(1, 3) tbl(i);

Casting Rules

Not all casts are possible. For example, it is not possible to convert an INTEGER to a DATE. Casts may also throw errors when the cast could not be successfully performed. For example, trying to cast the string 'hello' to an INTEGER will result in an error being thrown.

SELECT CAST('hello' AS INTEGER);

Conversion Error:
Could not convert string 'hello' to INT32

The exact behavior of the cast depends on the source and destination types. For example, when casting from VARCHAR to any other type, the string will be attempted to be converted.

TRY_CAST

TRY_CAST can be used when the preferred behavior is not to throw an error, but instead to return a NULL value. TRY_CAST will never throw an error, and will instead return NULL if a cast is not possible.

SELECT TRY_CAST('hello' AS INTEGER) AS i;

cast_to_type Function

The cast_to_type function allows generating a cast from an expression to the type of another column. For example:

SELECT cast_to_type('42', NULL::INTEGER) AS result;

┌───────┐
│  res  │
│ int32 │
├───────┤
│  42   │
└───────┘

This function is primarily useful in macros, as it allows you to maintain types. This helps with making generic macros that operate on different types. For example, the following macro adds to a number if the input is an INTEGER:

CREATE TABLE tbl(i INT, s VARCHAR);
INSERT INTO tbl VALUES (42, 'hello world');

CREATE MACRO conditional_add(col, nr) AS
    CASE
        WHEN typeof(col) == 'INTEGER' THEN cast_to_type(col::INTEGER + nr, col)
        ELSE col
    END;
SELECT conditional_add(COLUMNS(*), 100) FROM tbl;

┌───────┬─────────────┐
│   i   │      s      │
│ int32 │   varchar   │
├───────┼─────────────┤
│  142  │ hello world │
└───────┴─────────────┘

Note that the CASE statement needs to return the same type in all code paths. We can perform the addition on any input column by adding a cast to the desired type – but we need to cast the result of the addition back to the source type to make the binding work.