Pre-Conditions and Post-Conditions
The Rules Engine uses pre-conditions and post-conditions in some optimization techniques. These conditions are used in corrective UPDATE constraints.
A constraint has a pre-condition for a given column if, in the WHERE clause of the statement, the column is compared to a constant, or to an expression evaluating to a constant, or to NULL or NOT NULL. Clauses with subqueries and clauses containing OR are never pre-conditions. There may be more than one pre-condition: in this case the pre-conditions are linked by AND.
A constraint has a post-condition for a given column if that column is SET to a constant value, or to an expression evaluating to a constant, or to NULL. A SET clause setting the column to a subquery is never a post-condition. There may be several post-conditions: in this case the SET clause contains two or more parts separated by commas.
Consider the following SQL statement:
SET ColA =
(SELECT ... FROM <Table2> ... )
, ColB = 'N'
WHERE ColA IS NULL
AND ColC = 'Y'
In this SQL statement, the clauses:
ColA IS NULL
ColC = 'Y'
are pre-conditions, while the clause:
ColB IS 'N'
is a post-condition.
In SQL, you can write more than one SET statement using commas as separators as in the example, but you can use the following construct against Oracle only:
...SET (ColA, ColB) = (<Expr>,<Expr>)
Before corrective UPDATE constraints are evaluated, the Rules Engine looks after all pre-conditions and post-conditions of the constraints: