ocp 11g考試題庫詳解：1Z0-051-108題
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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
試題解析：
COUNT函數有以下三種格式：
COUNT(*)
COUNT(expr)
COUNT(DISTINCT expr)
COUNT(*)用于返回表中符合SELECT語句標準的行數，包括重復行和在任何列中含有空值的行。
如果SELECT語句中包含WHERE子句，則COUNT(*)會返回符合WHERE子句中條件的行數。
相反，COUNT(expr)返回由expr標識的列中非空值的數量。
COUNT(DISTINCT expr)返回由expr標識的列中不同非空值的數量。
