SQL SELECT. ( ), (join) . SQL. SELECT :
SELECT [ALL/DISTINCT] <_>
FROM <_>,..
[WHERE <> ]
[GROUP BY <_>,..]
[HAVING <>]
[ORDER BY <_>[ASC/DESC],]
SELECT 6 (): SELECT, FROM, WHERE, GROUPBY, HAVING ORDERBY. SELECT FROM . 6 , : 1)FROM. . 2)WHERE. , . 3)GROUPBY. , . 4)HAVING. , . 5)SELECT. , . 6)ORDERBY. . SELECT .
20.- , , , . . , , "" . "" .
21. :
1) - , A B, , , A, B, .
2) - , A B, , , A B.
3) - , A B, , , A B.
4) - (A1, A2, , Am, B1, B2, , Bm), A(A1, A2, , Am) B(B1, B2, , Bm), , A B:(a1, a2, , am, b1, b2, , bm) , (a1, a2, , am)∈A,(b1, b2, , bm)∈B.
5) - , A, , , c . c , A / .
6) - (X, Y, , Z) , (x, y, , z), , A X x, Y y, , Z z. - -.
7) - . , .
8) - (X1, X2, , Xn) , (x1, x2, , xn), , (y1, y2, , ym) ∈ B A(X1, X2, , Xn, Y1, Y2, , Ym) (x1, x2, , xn, y1, y2, , ym).