判断题（Judgement Questions）

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1. There can be multiple foreign keys in a relational schema. ( √ )

2. In SELECT statements, you can use ‘%’ to represent a string of arbitrary length (length can be 0). (√ )

3. Candidate keys for relational schemas can be one or more, but primary keys can only be one. ( √ )

4. A column in a relational table is also called a tuple. (× )

5. A row in a relational table is also called a property. (× )

6. SQL is a standardized query language. (× )

7. In SQL query commands, you can use the keyword DISTINCT to remove duplicate rows from the query results. (√ )

8. In a SQL query command, SUM function should be used to count the number of values in a column. (× )

9. In a SQL query command, COUNT function can be used in a SELECT clause or a HAVING clause, it can’t be used in a WHERE clause. (√ )

10. When using UPDATE statement in an SQL command to modify the data in the table, the corresponding clause to be used is SET clause. (√ )

11. Using views can speed up the execution of query statements. (× )

12. All kinds of users and programs can use all kinds of data in the database at will. (× )

13. Views only store definitions, not real data. (√ )

14. Index can improve the efficiency of a query, so we should create as much as indexes. (× )

15. Primary key constraints can be null but cannot be duplicate. (× )

16. Using * in SELECT statements to indicate that all attributes are selected. (√ )

17. Suppose that table student contains all student information， then the SQL statement “SELECT COUNT(*) FROM student; ”can check the total number of all the students。 (√ )

18. < DROP Tablename> will delete tables that already exist in the database. (× )

19. In a three-level database architecture, there are usually one conceptual schema, one internal schema and one external schema. (× )

20. The syntax for deleting all data from Student table is DROP FROM Student. (× )

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