, , , , .
WHERE : . :
- (a comp-op b), i>a i>b ; i>a i>b , i>comp-op, , ;
- (a comp-op const), a , const ; a , comp-op.
comp-op =, , >, , <, . (. . , 3), , -, -, true. , , unknown, .
A WHERE comp, A , comp . comp1 comp2 . :
- A WHERE (comp1 AND comp2) , (A WHERE comp1) INTERSECT (A WHERE comp2);
- A WHERE (comp1 OR comp2) , (A WHERE comp1) UNION (A WHERE comp2);
- A WHERE NOT comp1 , A MINUS (A WHERE comp1).
, , AND, OR, NOT .
___1 WHERE (_ > 20000.00 AND (__ = 310 OR __ = 315)) ( ___1 , 310 315 , 20 000.00 .) . 4.5.
. 3.5. ___1 WHERE (_ 200 > 20000.00 AND (__ = 310 OR __ = 315))
- ( ).
A , A.
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A { a 1, a 2,..., a n}(PROJECT A { a 1, a 2,..., a n}) , { a 1, a 2,..., a n}, , < a 1:v1, a 2:v2,..., a n:vn> , A , a 1 v1, a 2 v2,..., a n vn. , - -.
, , . , , , . ( .)
PROJECT ___1 {__} ( , ___1?) . 4.6.
. 3.6. PROJECT ___1 {__}