OCP-1Z0-新051-61题版本-34

QUESTION NO: 34

Which two statements are true regarding the COUNT function? (Choose two.)

A. COUNT(*) returns the number of rows including duplicate rows and rows containing NULL value in any of the columns

B. COUNT(cust_id) returns the number of rows including rows with duplicate customer IDs and NULL value in the CUST_ID column

C. COUNT(DISTINCT inv_amt) returns the number of rows excluding rows containing duplicates and NULL values in the INV_AMT column

D. A SELECT statement using COUNT function with a DISTINCT keyword cannot have a WHERE clause

E. The COUNT function can be used only for CHAR, VARCHAR2 and NUMBER data types

Answer: A,C

答案解析：

参考：108：http://blog.csdn.net/rlhua/article/details/11973833

Explanation:

Using the COUNT Function

The COUNT function has three formats:

COUNT(*)

COUNT(expr)

COUNT(DISTINCT expr)

COUNT(*) returns the number of rows in a table that satisfy the criteria of the SELECT statement,

including duplicate rows and rows containing null values in any of the columns. If a WHERE

clause is included in the SELECT statement, COUNT(*) returns the number of rows that satisfy the

condition in the WHERE clause.

In contrast,

COUNT(expr) returns the number of non-null values that are in the column identified by expr.

COUNT(DISTINCT expr) returns the number of unique, non-null values that are in the column

identified by expr.