PostgreSQL COUNT 函数
COUNT
COUNT() 函数返回匹配指定条件的行数。
如果指定的条件是列名，则 COUNT() 函数返回具有该名称的列数。
注意： NULL 值不会被计算在内。
通过指定 WHERE 子句，您可以例如返回来自伦敦的客户数量
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