题目内容

in SQL , the WHERE statement corresponds to which operator inrelational algebra

A. PROJECTION
B. SELECTION
C. UNION
D. INTERSECTION

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Which word can remove duplicate tuples from the query result

A. UNIQUE
B. COUNT
C. DISTINCT
D. UNION

in the following aggregate functions, Null values are not ignored in

A. SUM(COLUMN NAME)
B. MAX(COLUMN NAME)
C. AVG(COLUMN NAME)
D. COUNT(*)

In the SQL language, which of the following operations involving null values are incorrect

AGE IS NULL
B. AGE IS NOT NULL
C. AGE=NULL
D. NOT(AGE IS NULL)

DEPT(DNO,DNAME), if you want to find a DNAME whose third letter from the end is W and contains at least four letters, the query condition should be written as WHERE DNAME LIKE

A. '_ _W _%'
B. '_ W _ %'
C. '_ W _ _'
D. ' _ %W _ _'