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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);
i |
---|
1 |
2 |
3 |
SELECT i::DOUBLE AS i FROM generate_series(1, 3) tbl(i);
i |
---|
1.0 |
2.0 |
3.0 |
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;
i |
---|
NULL |