OCP-1Z0-051-2015-34题

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
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.   count(*)时，返回所有的行数，包括重复的，以及有空值的
In contrast,

COUNT(expr) returns the number of non-null values that are in the column identified by expr.返回由expr标识的列中非空值的数量。

COUNT(DISTINCT expr) returns the number of unique, non-null values that are in the column identified by expr.返回由expr标识的列中不同非空值的数量。

COUNT函数有以下三种格式：

• COUNT(*)

• COUNT(expr)

• COUNT(DISTINCT expr)

COUNT(*)用于返回表中符合SELECT语句标准的行数，包括重复行和在任何列中含有空值的行。

如果SELECT语句中包含WHERE子句，则COUNT(*)会返回符合WHERE子句中条件的行数。

相反，COUNT(expr)返回由expr标识的列中非空值的数量。

COUNT(DISTINCT expr)返回由expr标识的列中不同非空值的数量。

COUNT returns the number of rows returned by the query. You can use it as an aggregate or analytic function.你可以用它作为一个聚合函数或解析