SQL Count()

count(1), count(*),count(列名) 的区别

Demo
CREATE TABLE TEST_COUNT
  (
    "NAME" VARCHAR2(20 BYTE),
    "MAIL" VARCHAR2(20 BYTE)
  )

insert into TEST_COUNT(NAME, mail) values('a', '[e-mail]');
insert into TEST_COUNT(NAME, mail) values('b', '[e-mail]');
insert into TEST_COUNT(NAME, mail) values('c', '[e-mail]');
insert into TEST_COUNT(NAME, mail) values('d', '');
insert into TEST_COUNT(NAME) values('e');

select * from test_count;
select count(1),count(2),count(*),count(mail) from test_count;

结论
count(*)统计每一列
count(1)统计新添加列名为1
conun(列名）统计当前列名的列，注意如果此列为null，则不记数