Using Built-in Casts

User informix owns built-in casts. They govern conversions from one built-in data type to another. Built-in casts allow the database server to attempt the following data-type conversions:
  • A character type to any other character type
  • A character type to or from another built-in type
  • A numeric type to any other numeric type

The database server automatically invokes appropriate built-in casts when required. For time data types, conversion between DATE and DATETIME data types requires explicit casts with the EXTEND function, and explicit casts with the UNITS operator are required for number-to-INTERVAL conversion. Built-in casts are unavailable for converting large (BYTE, BLOB, CLOB, and TEXT) built-in types to other built-in data types.

When you convert a column from one built-in data type to another, the database server applies the appropriate built-in casts to each value already in the column. If the new data type cannot store any of the resulting values, the ALTER TABLE statement fails.

For example, if you try to convert a column from the INTEGER data type to the SMALLINT data type and the following values exist in the INTEGER column, the database server does not change the data type, because SMALLINT columns cannot accommodate numbers greater than 32,767: 
100    400    700    50000      700

The same situation might occur if you attempt to transfer data from FLOAT or SMALLFLOAT columns to INTEGER, SMALLINT, or DECIMAL columns. Errors of overflow, underflow, or truncation can occur during data type conversion.

Sections that follow describe database server behavior during certain types of casts and conversions.