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WHERE???????Join?????????????????
-- ??JoinSELECT * FROM tbl_emp a INNER JOIN tbl_dept b ON a.deptId = b.id;-- ?JoinSELECT * FROM tbl_emp a LEFT JOIN tbl_dept b ON a.deptId = b.id;-- ?JoinSELECT * FROM tbl_emp a RIGHT JOIN tbl_dept b ON a.deptId = b.id;-- ????MySQL????????Union?SELECT * FROM tbl_emp a UNION SELECT * FROM tbl_emp a WHERE a.deptId IS NULL;
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??????IO?????CPU??????????????????B+????????????
B+????????????IO??????????
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??EXPLAIN??????????????
type??possible_keys?key??rows??Extra??-- ????EXPLAIN SELECT id, author_id FROM article WHERE category_id = 1 AND comments > 1 ORDER BY views LIMIT 1;-- ????CREATE INDEX idx_article_ccv ON article(category_id, views);EXPLAIN SELECT id, author_id FROM article WHERE category_id = 1 AND comments > 1 ORDER BY views LIMIT 1;
-- ????EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card = book.card;-- ????CREATE INDEX idx_book_card ON book(card);CREATE INDEX idx_class_card ON class(card);EXPLAIN SELECT * FROM class LEFT JOIN book ON class.card = book.card;
CONCAT?SUBSTRING?CAST?CONVERT?> 1????????LIKE?????%???LIKE?????????WHERE??????????LIKE???OR????????????????????????????????????????
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