下列查询B=“大数据”且F=“开发平台”，结果集属性列为A、B、C、F的关系代数表达式中，查询效率最高的是（ ）。
- A.π1,2,3,8（σ2=‘大数据’^1=5^3=6^8=‘开发平台’（R×S））
- B.π1,2,3,8（σ1=5^3=6^8=‘开发平台’（σ2=‘大数据’（R）×S））
- C.π1,2,3,8（σ2=‘大数据’^1=5^3=6（R×σ4=‘开发平台’（S））
- D.π1,2,3,8（σ1=5^3=6（σ2=‘大数据’（R）×σ4=‘开发平台’（S）））
正确答案及解析
正确答案
解析
优化SQL语句，减少比较次数是提高查询效率的有效方法。
在这个题目中，如果连接的两个表越小，那么连接的时候多余的数据就更少，D答案将可以对子表做的操作先做了，最后做连接，是效率最高的一种方法。
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