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smallkun 2025-02-28 10:29:56 +08:00
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commit 1a337cdb21
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2208/天天乐学考试答案/MySQL练习（一）.md
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@ -248,7 +248,7 @@ CREATE VIEW v_book AS
SELECT b.genre, a.author_name, COUNT(*) c SELECT b.genre, a.author_name, COUNT(*) c
FROM tb_books b, tb_authors a FROM tb_books b, tb_authors a
WHERE b.note = a.author_name WHERE b.note = a.author_name
GROUP BY b.genre, a.author_id GROUP BY b.genre, a.author_id #按照图书类别和作者名称进行分组
HAVING COUNT(*) = ( #子查询用来统计这个类别中 数量最多的作者的图书数量 HAVING COUNT(*) = ( #子查询用来统计这个类别中 数量最多的作者的图书数量
SELECT COUNT(*) SELECT COUNT(*)
FROM tb_books FROM tb_books
@ -258,6 +258,34 @@ HAVING COUNT(*) = ( #子查询用来统计这个类别中 数量最多的作者
LIMIT 1 LIMIT 1
); );
#10
DELIMITER $$
DROP TRIGGER IF EXISTS tri_updateCentury;
CREATE TRIGGER tri_updateCentury
BEFORE UPDATE ON tb_books
FOR EACH ROW
BEGIN
#方式一
/*
计算出世纪数+字符串拼接
IF NEW.publication_date <> OLD.publication_date THEN
SET NEW.century = CONCAT(FLOOR(YEAR(NEW.publication_date)/100) + 1, '世纪');
END IF;
*/
#方式二 使用多分支IF
IF NEW.publication_date <> OLD.publication_date THEN
IF YEAR(NEW.publication_date) BETWEEN 1800 AND 1899 THEN
SET NEW.century = '19世纪';
ELSEIF YEAR(NEW.publication_date) BETWEEN 1900 AND 1999 THEN
SET NEW.century = '20世纪';
ELSEIF YEAR(NEW.publication_date) BETWEEN 2000 AND 2099 THEN
SET NEW.century = '21世纪';
END IF;
END IF;
END $$
DELIMITER ;
SELECT CONCAT(FLOOR(YEAR(publication_date)/100) + 1, '世纪')
FROM tb_books;
``` ```