Casting between data types

There are many occasions when a value with a given data type needs to be cast (changed) to a different data type or to the same data type with a different length, precision, or scale. Data type promotion, as described in Promotion of data types, is one example of when a value with one data type needs to be cast to a new data type. A data type that can be changed to another data type is castable from the source data type to the target data type.

The casting of one data type to another can occur implicitly or explicitly. The cast functions or CAST specification (see CAST specification) can be used to explicitly change a data type. The database manager might implicitly cast data types during assignments that involve a distinct type (see Distinct type assignments). In addition, when you create a sourced user-defined function, the data types of the parameters of the source function must be castable to the data types of the function that you are creating (see CREATE FUNCTION (Sourced)).

If truncation occurs when a character or graphic string is cast to another data type, a warning occurs if any non-blank characters are truncated. This truncation behavior is similar to retrieval assignment of character or graphic strings (see Retrieval assignment).

For casts that involve a distinct type as either the data type to be cast to or from, Table 12 shows the supported casts. For casts between built-in data types, Table 13 shows the supported casts.

Table 12. Supported Casts When a Distinct Type is Involved
Data Type ...	Is Castable to Data Type ...
Distinct type DT	Source data type of distinct type DT
Source data type of distinct type DT	Distinct type DT
Distinct type DT	Distinct type DT
Data type A	Distinct type DT where A is promotable to the source data type of distinct type DT (see Promotion of data types)
INTEGER	Distinct type DT if DT's source type is SMALLINT
DOUBLE	Distinct type DT if DT's source data type is REAL
VARCHAR	Distinct type DT if DT's source data type is CHAR or GRAPHIC
VARGRAPHIC	Distinct type DT if DT's source data type is GRAPHIC or CHAR
VARBINARY	Distinct type DT if DT's source data type is BINARY

When a distinct type is involved in a cast, a cast function that was generated when the distinct type was created is used. How the database manager chooses the function depends on whether function notation or the CAST specification syntax is used. For details, see Function resolution, and CAST specification. Function resolution is used for both. However, in a CAST specification, when an unqualified distinct type is specified as the target data type, the database manager resolves the schema name of the distinct type and then uses that schema name to locate the cast function.

The following table describes the supported casts between built-in data types.

Table 13. Supported Casts Between Built-In Data Types
Target Data Type → Source Data Type ↓	SMALLINT INTEGER BIGINT	DECIMAL NUMERIC	REAL DOUBLE	CHAR VARCHAR CLOB	GRAPHIC VARGRAPHIC DBCLOB	BINARY VARBINARY BLOB	DATE	TIME	TIMESTAMP	ROWID	DATALINK
SMALLINT	Y	Y	Y	Y	Y¹	—	—	—	—	—	—
INTEGER	Y	Y	Y	Y	Y¹	—	—	—	—	—	—
BIGINT	Y	Y	Y	Y	Y¹	—	—	—	—	—	—
DECIMAL	Y	Y	Y	Y	Y¹	—	—	—	—	—	—
NUMERIC	Y	Y	Y	Y	Y¹	—	—	—	—	—	—
REAL	Y	Y	Y	Y	Y¹	—	—	—	—	—	—
DOUBLE	Y	Y	Y	Y	Y¹	—	—	—	—	—	—
CHAR	Y	Y	Y	Y	Y¹	Y	Y	Y	Y	Y	—
VARCHAR	Y	Y	Y	Y	Y¹	Y	Y	Y	Y	Y	—
CLOB	Y	Y	Y	Y	Y¹	Y	Y	Y	Y	Y	—
GRAPHIC	Y¹	Y¹	Y¹	Y¹	Y	Y	Y¹	Y¹	Y¹	—	—
VARGRAPHIC	Y¹	Y¹	Y¹	Y¹	Y	Y	Y¹	Y¹	Y¹	—	—
DBCLOB	Y¹	Y¹	Y¹	Y¹	Y	Y	Y¹	Y¹	Y¹	—	—
BINARY	—	—	—	—	—	Y	—	—	—	—	—
VARBINARY	—	—	—	—	—	Y	—	—	—	—	—
BLOB	—	—	—	—	—	Y	—	—	—	—	—
DATE	—	—	—	Y	Y¹	—	Y	—	Y	—	—
TIME	—	—	—	Y	Y¹	—	—	Y	Y	—	—
TIMESTAMP	—	—	—	Y	Y¹	—	Y	Y	Y	—	—
ROWID	—	—	—	Y	—	Y	—	—	—	Y	—
DATALINK	—	—	—	—	—	—	—	—	—	—	Y
Notes: ¹ Conversion is only supported for UTF-16 or UCS-2 graphic.

The following table describes the rules for casting to a data type:

Table 14. Rules for Casting to a Data Type
Target Data Type	Rules
SMALLINT	See SMALLINT.
INTEGER	See INTEGER or INT.
BIGINT	See BIGINT.
DECIMAL	See DECIMAL or DEC.
NUMERIC	See ZONED.
REAL	See REAL.
DOUBLE	See DOUBLE_PRECISION or DOUBLE.
CHAR	See CHAR.
VARCHAR	See the VARCHAR.
CLOB	See CLOB.
GRAPHIC	If the source data type is a character string, see the rules for string assignment to a variable in Assignments and comparisons. Otherwise, see GRAPHIC.
VARGRAPHIC	If the source data type is a character string, see the rules for string assignment to a variable in Assignments and comparisons. Otherwise, see VARGRAPHIC.
DBCLOB	See DBCLOB.
BINARY	See BINARY.
VARBINARY	See VARBINARY.
BLOB	See BLOB.
DATE	See DATE.
TIME	See TIME.
TIMESTAMP	If the source data type is a string, see TIMESTAMP, where one operand is specified. If the source data type is a DATE, the timestamp is composed of the specified date and a time of 00:00:00. If the source data type is a TIME, the timestamp is composed of the CURRENT_DATE and the specified time.
DATALINK	See the rules for DataLink assignments in Assignments and comparisons.
ROWID	See ROWID.

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