SQL:COUNT()函数
COUNT()函数返回匹配指定条件的行数
COUNT(column_name):返回指定列的值的数目(NULL不计入)
例子:
SELECT COUNT(Customer/*列名*/) AS CustomerNilsen/*输出列名*/ FROM Orders/*查找表名*/
WHERE Customer='Carter'
输出:
COUNT(DISTINCT column_name):返回指定列的不同值的数目
例子:
SELECT COUNT(DISTINCT Customer) AS NumberOfCustomers FROM Orders
输出:
COUNT(*):返回表中的记录数
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