问题
If i have a column say column a of any given values, and i want another column column b to have a default value according to the value of column a
In another words:
if column a = 'peter' then column b default value = 'doctor'.
回答1:
This is not possible with a simple DEFAULT value, as the manual clearly states:
The value is any variable-free expression (subqueries and cross-references to other columns in the current table are not allowed).
You could use a trigger instead:
CREATE OR REPLACE FUNCTION trg_foo_b_default()
RETURNS trigger AS
$func$
BEGIN
-- For just a few constant options, CASE does the job:
NEW.b :=
CASE NEW.a
WHEN 'peter' THEN 'doctor'
WHEN 'weirdo' THEN 'shrink'
WHEN 'django' THEN 'undertaker'
ELSE NULL
END;
/* -- For more, or dynamic options, you could use a lookup table:
SELECT INTO NEW.b t.b
FROM def_tbl t
WHERE t.a = NEW.a;
*/
RETURN NEW;
END
$func$ LANGUAGE plpgsql;
CREATE TRIGGER b_default
BEFORE INSERT ON foo
FOR EACH ROW
WHEN (NEW.b IS NULL AND NEW.a IS NOT NULL)
EXECUTE PROCEDURE trg_foo_b_default();
To make this more effective I use a WHEN clause (available since Postgres 9.0) with the trigger definition. This way the trigger function is only executed, when it's actually useful. I am assuming here, we can let b IS NULL slide if a IS NULL.
Works in a similar, but subtly different fashion from a DEFAULT value.
With a default value, you can explicitly insert NULL to overrule the default. That's not possible here, NULL in b is replaced with the value derived from a.
来源：https://stackoverflow.com/questions/16737738/postgresql-set-a-default-cell-value-according-to-another-cell-value