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71 lines
2.1 KiB
Plaintext
71 lines
2.1 KiB
Plaintext
----------------
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Identity Columns
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----------------
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Author:
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Adriano dos Santos Fernandes <adrianosf@gmail.com>
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Description:
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An identity column is a column associated with an internal sequence generator and has it value
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automatically set when omitted in an INSERT statement.
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Syntax:
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<column definition> ::=
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<name> <type> GENERATED BY DEFAULT AS IDENTITY [ (START WITH <value>) ] <constraints>
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<alter column definition> ::=
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<name> RESTART [ WITH <value> ]
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Syntax rules:
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- The type of an identity column must be an exact number type with zero scale. That includes:
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smallint, integer, bigint, numeric(x, 0) and decimal(x, 0).
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- Identity columns can't have DEFAULT or COMPUTED value.
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Notes:
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- You cannot alter a identity column to normal column and vice versa.
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- Identity columns are implicitly NOT NULL.
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- Identity columns don't enforce uniqueness automatically. Use UNIQUE or PRIMARY key for that.
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Implementation:
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Two columns have been inserted in RDB$RELATION_FIELDS: RDB$GENERATOR_NAME and RDB$IDENTITY_TYPE.
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RDB$GENERATOR_NAME stores the automatically created generator for the column. In RDB$GENERATORS,
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the value of RDB$SYSTEM_FLAG of that generator will be 6. RDB$IDENTITY_TYPE will currently
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always store the value 1 (by default) for identity columns and NULL for non-identity columns.
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In the future this column will store the value 0, too (for ALWAYS) when Firebird support this type
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of identity column.
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Example:
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create table objects (
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id integer generated by default as identity primary key,
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name varchar(15)
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);
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insert into objects (name) values ('Table');
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insert into objects (name) values ('Book');
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insert into objects (id, name) values (10, 'Computer');
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select * from objects order by id;
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commit;
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ID NAME
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============ ===============
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1 Table
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2 Book
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10 Computer
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alter table objects
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alter id restart with 14;
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insert into objects (name) values ('Pencil');
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select * from objects order by id;
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ID NAME
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============ ===============
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1 Table
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2 Book
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10 Computer
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15 Pencil
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