CHECK
Constraint
The CHECK
constraint defines the condition the values inserted in this column or row must satisfy.A condition is a logical expression (also called a predicate) that can return the TRUE
, FALSE
and UNKNOWN
values.A condition is considered satisfied if the predicate returns TRUE
or value UNKNOWN
(equivalent to NULL
).If the predicate returns FALSE
, the value will not be accepted.This condition is used for inserting a new row into the table (the INSERT
statement) and for updating the existing value of the table column (the UPDATE
statement) and also for statements where one of these actions may take place (UPDATE OR INSERT
, MERGE
).
Important

A 
CHECK
constraints — whether defined at table level or column level — refer to table columns by their names.The use of the keyword VALUE
as a placeholder — as in domain CHECK
constraints — is not valid in the context of defining constraints in a table.
with two columnlevel constraints and one at tablelevel:
CREATE TABLE PLACES (
...
LAT DECIMAL(9, 6) CHECK (ABS(LAT) <= 90),
LON DECIMAL(9, 6) CHECK (ABS(LON) <= 180),
...
CONSTRAINT CHK_POLES CHECK (ABS(LAT) < 90 OR LON = 0)
);