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山东理工大学/其他/lab4.md

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+CREATE PROCEDURE RateModel ( IN cid VARCHAR ( 20 ), IN m VARCHAR ( 20 ), IN r INT ) BEGIN-- 1. 插入新评分记录
+  INSERT INTO ratings ( customer_id, model, rating_time, rating )
+  VALUES
+    ( cid, m, NOW( ), r ) -- 2. 更新 Products 表的总评分和评分次数
+    INSERT IGNORE INTO Products ( total_rating, number_of_ratings )
+  VALUES
+    ( 0, 0 );-- 3. 计算该型号的最新总评分和评分次数
+  UPDATE products 
+  SET total_rating = ( SELECT COALESCE( SUM( rating ), 0 ) FROM Ratings WHERE model = m ),
+  number_of_ratings = ( SELECT COUNT( * ) FROM Ratings WHERE model = m ) 
+  WHERE
+    model = m;
+
+  END -- 
+-- 2
+  CREATE PROCEDURE DeleteRating ( IN cid ( 10 ), IN m ( 10 ), ) BEGIN
+    CALL RateModel ( p_customer_id, p_model, NULL );
+    
+    END -- 3
+    CREATE PROCEDURE CleanRatings ( INT cid ( 10 ) INT m ( 10 ) ) BEGIN
+      DELETE 
+      FROM
+        Rating 
+      WHERE
+        customer_id = cid 
+        AND model = m 
+        AND rating_time < (
+          SELECT
+            max_time 
+          FROM
+            ( SELECT MAX( r2.rating_time ) AS max_time FROM Ratings AS r2 WHERE r2.customer_id = p_customer_id AND r2.model = p_model ) AS t 
+        );
+      
+    END;
+    CALL RateModel ( '1122334455', '1001', 3 );
+    CALL RateModel ( '1122334455', '1001', 2 );
+    CALL RateModel ( '9999999999', '1001', 5 );
+    CALL RateModel ( '1122334455', '1001', NULL );
+    CALL RateModel ( '9999999999', '1001', 4 );
+    CALL RateModel ( '9999999999', '1001', NULL );
+    CALL RateModel ( '1122334455', '1001', 2 );
+  CALL CleanRatings ( '9999999999', '1001' );
+    CALL CleanRatings ( '1122334455', '1001' );
+    
+    
+    
+    
+--     3
+CREATE VIEW AverageRatings(model, average_rating)
+AS
+SELECT 
+    model,
+    COALESCE(SUM(rating) / COUNT(*), 0) AS average_rating
+FROM Ratings
+GROUP BY model
+HAVING COUNT(*) > 0; -- 仅包含至少有1次评分的型号
+
+
+-- 3.b
+DELIMITER //
+CREATE FUNCTION BestModel()
+RETURNS VARCHAR(20)
+DETERMINISTIC
+BEGIN
+    DECLARE max_avg FLOAT;
+    DECLARE best_model VARCHAR(20);
+    
+    -- 步骤1：获取最高平均评分
+    SELECT MAX(average_rating) INTO max_avg FROM AverageRatings;
+    
+    -- 步骤2：若无评分，返回NULL
+    IF max_avg IS NULL THEN
+        RETURN NULL;
+    END IF;
+    
+    -- 步骤3：获取平均评分最高的所有型号，选择最新非空评分的型号
+    SELECT model INTO best_model
+    FROM (
+        -- 子查询：获取平均评分最高的型号，及其最新非空评分时间
+        SELECT 
+            ar.model,
+            MAX(CASE WHEN r.rating IS NOT NULL THEN r.rating_time ELSE NULL END) AS latest_non_null_time
+        FROM AverageRatings ar
+        JOIN Ratings r ON ar.model = r.model
+        WHERE ar.average_rating = max_avg
+        GROUP BY ar.model
+        ORDER BY latest_non_null_time DESC -- 按最新非空评分时间降序
+        LIMIT 1 -- 取最新的一个
+    ) AS temp;
+    
+    RETURN best_model;
+END //
+DELIMITER ;
+
+
+-- 3.c