Oracle_Count函数
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COUNT
Syntax
Description of the illustration count.gif
See Also:
"Analytic Functions" for information on syntax, semantics, and restrictionsPurpose
COUNT
returns the number of rows returned by the query. You can use it as an aggregate or analytic function.
If you specify DISTINCT
, then you can specify only the query_partition_clause
of theanalytic_clause
. The order_by_clause
and windowing_clause
are not allowed.
If you specify expr
, then COUNT
returns the number of rows whereexpr
is not null. You can count either all rows, or only distinct values ofexpr
.
If you specify the asterisk (*), then this function returns all rows, including duplicates and nulls.COUNT
never returns null.
See Also:
"About SQL Expressions" for information on valid forms ofexpr
and "Aggregate Functions"Aggregate Examples
The following examples use COUNT
as an aggregate function:
SELECT COUNT(*) "Total" FROM employees; Total---------- 107SELECT COUNT(*) "Allstars" FROM employees WHERE commission_pct > 0; Allstars--------- 35SELECT COUNT(commission_pct) "Count" FROM employees; Count---------- 35SELECT COUNT(DISTINCT manager_id) "Managers" FROM employees; Managers---------- 18
Analytic Example
The following example calculates, for each employee in the employees
table, the moving count of employees earning salaries in the range 50 less than through 150 greater than the employee's salary.
SELECT last_name, salary, COUNT(*) OVER (ORDER BY salary RANGE BETWEEN 50 PRECEDING AND 150 FOLLOWING) AS mov_count FROM employees ORDER BY salary, last_name;LAST_NAME SALARY MOV_COUNT------------------------- ---------- ----------Olson 2100 3Markle 2200 2Philtanker 2200 2Gee 2400 8Landry 2400 8Colmenares 2500 10Marlow 2500 10Patel 2500 10
SQL> select comm from emp; COMM---------- 300 500 1400 0 COMM----------14 rows selected.SQL> select distinct comm from emp; COMM---------- 1400 500 300 05 rows selected.
SQL> select comm from emp; COMM---------- 300 500 1400 0 COMM----------14 rows selected.SQL> select distinct comm from emp; COMM---------- 1400 500 300 05 rows selected.
SQL> select count(comm) from emp where comm is not null;COUNT(COMM)----------- 41 row selected.SQL> select count(comm) from emp where comm is null;COUNT(COMM)----------- 01 row selected.
总结:count(*)结果包含NULL行,count(某一列)不包含空行。
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