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                            最適合索引的列是出現在 WHERE 子句中的列

MySQL 前綴索引能有效減小索引文件的大小，提高索引的速度。但是前綴索引也有它的壞處：MySQL 不能在 ORDER BY 或 GROUP BY 中使用前綴索引，也不能把它們用作覆蓋索引(Covering Index)。
# 示例
ALTER TABLE city ADD KEY(cityname(7));
                    
                
        
                
            
        
                
    
                
    
                
        
                
    
                

                







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