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                >[info]sql設計思考 >[success]## select 對等的業務輸出對等的字段數據信息 ,查詢數據時,只獲取必要字段。 >>>[info]某個業務只需要輸出用戶ID,用戶名和電話號碼 ,那么查詢的時候sql語句如下: ``` select `uid、name、userphone` from user ``` >>>[success]優點: * 節省cpu/io/內存/帶寬的消耗 * 指定字段能有效利用索引覆蓋 * 在表結構變更時,能保證對應用程序無影響 >[danger]>Tips: * 盡量用IN來替換OR * 盡量避免多表Join查詢 * 減少使用or語句,可將or語句優化為union,然后在各個where條件上建立索引。如`where a=1 or b=2`優化為`where a=1… union …where b=2, key(a),key(b)`。 *** <br> >[success] ## insert insert語句指定具體字段名稱,不要寫成`insert into t1 values(…)`
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