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.