请教几道经典SQL问题！谢谢！

1） Why would you want to sometimes de-normalize data?

A. Prevent data from becoming inconsistent

B. Improve query response

C. Eliminate redundant data

D. All of the above

2） Given two tables, each with one column X of type number which is the primary key. Table A has 100 rows, Table B has 50 rows. When doing an Inner Join between A and B on X, which option would be correct?

• The maximum possible number of rows is 100, the minimum possible number of rows 50

• The maximum possible number of rows is 5000, the minimum possible number of rows 0

• The maximum possible number of rows is 50, the minimum possible number of rows 0

• The maximum possible number of rows is 100, the minimum possible number of rows 100

• It is not possible to estimate a maximum and minimum. More information is needed.

  
  

3） Given two tables, each with one column X of type number which is the primary key. Table A has 100 rows, Table B has 50 rows. When doing a Left Outer join from A to B on X, which option would be correct?

• The maximum possible number of rows is 100, the minimum possible number of rows 50

• The maximum possible number of rows is 5000, the minimum possible number of rows 0

• The maximum possible number of rows is 50, the minimum possible number of rows 0

• The maximum possible number of rows is 100, the minimum possible number of rows 100

• It is not possible to estimate a maximum and minimum. More information is needed.

  
  

4）Two tables A and B are given:

A          B

X Y        Y Z

1 1        2 1

2 2        2 2  

3 1        3 3  

4 2        4 5

How many rows will be in the result set of the following query?

SELECT B.Z

FROM A LEFT OUTER JOIN B

ON A.X = B.Y

• 6 Rows

• 5 Rows

• 4 Rows

• 2 Rows

• 1 Row

  
  

5）Two tables A and B are given:

A         B

X         Y

1         1

2         2

3         5  

          7

How many rows will be in the result set of the following query?

SELECT A.X, B.Y

FROM A, B

WHERE A.X < 3 AND B.Y > = 2

• 5 Rows

• 6 Rows

• 7 Rows

• 10 Rows

• 12 Rows

  
  

6） The following EMPLOYEE table is given:

ID   NAME        DEPT    SALARY        

1    JOHN        HR      100

2    SAM         IT      200

3    TOM         QA      150

4    NICOLE

这种题目，建议楼主安装SQL环境实验，方知表之间集合关系。祝楼主学习SQL愉快。我来回答第4题，环境为SQL SERVER。

DECLARE @A TABLE(X INT,Y INT);
DECLARE @B TABLE(        Y INT ,Z INT);
INSERT INTO @A(X,Y) VALUES(1,1),(2,2),(3,1),(4,2);
INSERT INTO @B(Y,Z) VALUES(2,1),(2,2),(3,3),(4,5);
SELECT A.*,B.* FROM @A A LEFT OUTER JOIN @B B ON A.X = B.Y;
SELECT B.Z FROM @A A LEFT OUTER JOIN @B B ON A.X = B.Y;