nullif(var1,var2)
如果var1和var2相等则返回null,如果不相等则返回var1
select nullif(1,null); 结果:1
select nullif(null,1); 结果:null
select nullif(1,1); 结果:null
select nullif(1,0); 结果:1
create table posts(
id serial primary key,
title varchar(255) not null,
excerpt varchar(255),
body text,
create_tiem timestamp default current_timestamp,
update_time timestamp
);
insert into posts(title,excerpt,body) values
('test post 1','test post excerpt 1','test post body 1'),
('test post 2','','test post body 2'),
('test post 3', null ,'test post body 3');
案例1:
select id,title ,excerpt from posts;
结果:由结果得知excerpt有空值null
id title excerpt
1 test post 1 test post excerpt 1
2
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