在关系代数中，最常用的式子是“投影选择联接表达式” (41) ，此处凡“R1×R2×…×R2为基本表，F是公式，A1，A2，…，An为属性，针对这个表达式，SQL设计了SELECT句型：

 SELECT A1，A2，…．An

 FROM R1，R2，…Rn

 WHERE F

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(71)data　effectively　is　crucial　for　success　in　today's　competitive　environment.　Managers　must　know　how　to　use　a　variety　of　tools.　Integrated　data　takes　information　from　different　sources　and　puts　it　together　in　a　meaningful　and　useful　way.　One　of　the　difficulties　of　this　is　the(72)in　hardware　and　software.(73)integration　uses　a　base　document　that　contains　copies　of　other　objects.(74)integration　uses　a　base　document　that　contains　the　current　or　most　recent　version　of　the　source　document　it　contains.(75)provides　an　overview　of　the　program　written　in"　plain"　English,　without　the　computer　syntax.

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