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2208/天天乐学考试答案/MySQL练习（一）.md

@@ -248,7 +248,7 @@ CREATE VIEW v_book AS
 SELECT b.genre, a.author_name, COUNT(*) c
 FROM tb_books b, tb_authors a
 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
 	SELECT COUNT(*)
 	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
 );
 
+#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;
 ```