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Casting to TYPE OF COLUMN

When operands are cast to the type of a column, the specified column may be from a table or a view.

Only the type of the column itself is used.For character types, the cast includes the character set, but not the collation.The constraints and default values of the source column are not applied.

Example
CREATE TABLE TTT (
  S VARCHAR (40)
  CHARACTER SET UTF8 COLLATE UNICODE_CI_AI
);
COMMIT;

SELECT
  CAST ('I have many friends' AS TYPE OF COLUMN TTT.S)
FROM RDB$DATABASE;

Conversions Possible for the CAST Function

Table 1. Conversions with CAST
From Data Type To Data Type

Numeric types

Numeric types, [VAR]CHAR, BLOB

[VAR]CHAR

[VAR]CHAR, BLOB, Numeric types, DATE, TIME, TIMESTAMP, BOOLEAN

BLOB

[VAR]CHAR, BLOB, Numeric types, DATE, TIME, TIMESTAMP, BOOLEAN

DATE, TIME

[VAR]CHAR, BLOB, TIMESTAMP

TIMESTAMP

[VAR]CHAR, BLOB, DATE, TIME

BOOLEAN

BOOLEAN, [VAR]CHAR, BLOB

To convert string data types to the BOOLEAN type, the value must be (case-insensitive) 'true' or 'false', or NULL.

Important

Keep in mind that partial information loss is possible.For instance, when you cast the TIMESTAMP data type to the DATE data type, the time-part is lost.