考試科目：1Z0-082

考試題量：90

通過(guò)分?jǐn)?shù)：60%

考試時(shí)間：150min

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24.Which three statements are true regarding the UNION and UNION ALL operators?

A.The number of columns selected by the first SELECT statement can be greater than the number selected in subsequent SELECT statements

B.Duplicates are eliminated automatically by the UNION ALL operator

C.The number of columns selected in each SELECT statement must be identical

D.NULIS are not ignored during duplicate checking

E.The names of columns selected in each SELECT statement must be identical

F.The names of columns selected in each SELECT statement can be identical

G.Duplicates can optionally be eliminated by the UNION operator

參考答案：CDF

解析：

A.錯(cuò):? 列數(shù)量應(yīng)該一致

B.錯(cuò)：union all不會(huì)刪除重復(fù)項(xiàng)

C.對(duì)：identical是相同的意思，集合操作列數(shù)應(yīng)該相同

D.對(duì)：空值不會(huì)被忽略，可以用SELECT NULL FROM dual UNION SELECT 1 FROM dual測(cè)試

E.錯(cuò)：列名可以不一樣，以第一個(gè)集合的列名為準(zhǔn)

F.對(duì)：列名可以相同也可以不同，以第一個(gè)SQL為準(zhǔn)

G.union 自動(dòng)消除重復(fù)項(xiàng)目

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