下面选项中，用于在删除存储过程时，检测存储过程是否存在的关键字是（）。 A: IF EXISTS B: HAS EXISTS C: AS EXISTS D: IS EXISTS

删除数据库的命令正确的是？ A: drop database if not exists test; B: drop database if exists test; C: drop database if exists not test; D: drop database test if exists ;

使用SQL语句删除表mytest,下列SQL语句正确的是( )。 A: DROP TABLE mytest IF EXISTS B: DROP TABLE IF EXISTS mytest C: ALTER TABLE IF EXISTS mytest ALTER TABLE mytest IF EXISTS

以下选项中哪些是存在谓词（） A: EXISTS B: NOT EXISTS C: EXPLAIN D: SELECT

When a free-rider problem exists,

The fact is ___ exists no life on the moon.

The "lemons problem" exists because of

查询专业不存在的学生信息，正确的命令是？ A: db.students.find({"major":null}) B: db.students.find({"major":{$in:[null],$exists:true}}) C: db.students.find({"major":{$exists:false}}) D: db.students.find({"major":{$exists:true}})

若要查询评论了所有电影的评论者name,则下列空白处需要依次填入——。SELECT name FROM Reviewer WHERE rID (SELECT * FROM Movie WHERE mID (SELECT mID FROM RatingWHERE Reviewer.rID=Rating.rID and Movie.mID=Rating.mID)) A: NOT IN, NOT IN B: NOT EXISTS, NOT EXISTS C: NOT IN, NOT EXISTS D: NOT EXISTS, NOT IN

The image of the bear mentioned in this film exists __________.