COUNT（*）是否包含空值？

COUNT（*）是否包含空值？

如果你有这张桌子

表格1：

 Field1    Field2    Field3 ---------------------------   1         1         1  NULL      NULL      NULL   2         2        NULL   1         3         1

然后

 SELECt COUNT(*), COUNT(Field1), COUNT(Field2), COUNT(DISTINCT Field3) FROM Table1

输出为：

 COUNT(*) = 4; -- count all rows, even null/duplicates -- count only rows without null values on that field COUNT(Field1) = COUNT(Field2) = 3 COUNT(Field3) = 2  COUNT(DISTINCT Field3) = 1 -- Ignore duplicates