OCP 11g考试题库详解：1Z0-051-108题
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108. Which two statements are true regarding the COUNT function? (Choose two.)
A. The COUNT function can be used only for CHAR, VARCHAR2, and NUMBER data types.
B. COUNT(*) returns the number of rows including duplicate rows and rows containing NULL value in
any of the columns.
C. COUNT(cust_id) returns the number of rows including rows with duplicate customer IDs and NULL
value in the CUST_ID column.
D. COUNT(DISTINCT inv_amt)returns the number of rows excluding排除在外 rows containing duplicates and
NULL values in the INV_AMT column.
E. A SELECT statement using the COUNT function with a DISTINCT keyword cannot have a WHERE clause.
Answer: BD
试题解析：
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。
