SQL. Вложенные запросы. Правила создания запросов

Правила создания запросов
…WHERE <поле> = (SELECT <поле> … )
SELECT * FROM PREDMET WHERE TNUM = (SELECT TFAM FROM TEACHERS WHERE TFAM = ‘Бахтин’);
SELECT * FROM PREDMET WHERE TNUM = (SELECT TFAM FROM TEACHERS WHERE TFAM = ‘Бахтин’);
SELECT * FROM PREDMET WHERE TNUM = (SELECT TNUM FROM TEACHERS WHERE TFAM = ‘Важенин’);

Правила создания запросов
…WHERE <поле> = (SELECT DISTINCT <поле> …)
SELECT * FROM PREDMET WHERE TNUM = (SELECT DISTINCT TNUM FROM TEACHERS WHERE TFAM = ‘Важенин’);
SELECT * FROM USP WHERE OCENKA > (SELECT AVG (OCENKA) FROM USP);

Правила создания запросов
SELECT OCENKA, COUNT (SNUM) FROM USP GROUP BY OCENKA
HAVING OCENKA =
(SELECT AVG(OCENKA) FROM USP WHERE PNUM=2003)