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106 lines
3.2 KiB
Plaintext
106 lines
3.2 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 { ALWAYS | BY DEFAULT } AS IDENTITY [ ( <identity column option>... ) ] <constraints>
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<identity column option> ::=
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START WITH <value> |
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INCREMENT [ BY ] <value>
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<alter column definition> ::=
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<name> <set identity column generation clause> [ <alter identity column option>... ] |
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<name> <alter identity column option>... |
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<name> DROP IDENTITY
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<set identity column generation clause> ::=
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SET GENERATED { ALWAYS | BY DEFAULT }
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<alter identity column option> ::=
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RESTART [ WITH <value> ] |
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SET INCREMENT [ BY ] <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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- Increment value cannot be 0.
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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 stores the value
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0 for GENERATED ALWAYS, 1 for GENERATED BY DEFAULT, and NULL for non-identity columns.
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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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alter table objects
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alter id set increment by 2;
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alter table objects
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alter id drop identity;
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---------------
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Override Clause
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---------------
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BY DEFAULT identity columns can be overriden in INSERT statements (INSERT, UPDATE OR INSERT, MERGE ... WHEN NOT MATCHED)
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just specifying the value in the values list. However, for ALWAYS identity columns that is not allowed.
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To use the value passed in the INSERT statement for an ALWAYS column, you should pass OVERRIDING SYSTEM VALUE as
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following:
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insert into objects (id, name) overriding system value values (11, 'Laptop');
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OVERRIDING also supports a subclause to ignore the value passed in INSERT and use the defined sequence:
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insert into objects (id, name) overriding user value values (12, 'Laptop'); -- 12 is not used
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