In-depth understanding of Null in SQL

NULL means unknown and uncertain in the computer and programming world. Although the Chinese translation is "empty", this void (null) is not that empty (empty). Null represents an unknown state, a future state. For example, I don’t know how much money Xiao Ming has in his pocket, but I can’t be sure it is 0. At this time, Null is used in computers to represent unknown and uncertain.

Although people who are proficient in SQL will not have any doubts about Null, it is still difficult to find an article that summarizes it comprehensively. I saw an English version and it felt good.

Tony Hoare invented the null reference in 1965, and considered it the "billion-dollar mistake" he made. Even today, 50 years later, null values ​​in SQL are still the culprit of many common errors.

Let’s take a look at some of the most shocking situations.

Null does not support size/equality judgment

For the following two queries, no matter how many records there are in the users table, the records returned are 0 rows:

select * from users where deleted_at = null;

– result: 0 rows

select * from users where deleted_at != null;

– result: 0 rows

How could this be? All because null represents an "unknown" type. That is, it makes no sense to use normal conditional operators to compare null with other values. Null is not equal to Null (approximate understanding: an unknown value cannot be equal to an unknown value, and the relationship between the two is also unknown, otherwise mathematics and logic will be messed up).

– Note: The following SQL is suitable for MySQL. If it is Oracle, you need to add... from dual;

select null > 0;

– result: null

select null < 0;

– result: null

select null = 0;

– result: null

select null = null;

– result: null

select null != null;

– result: null

The correct way to compare a value to null is to use the is keyword, and the is not operator:

select * from users

where deleted_at is null;

– result: 所有被标记为删除的 users

select * from users

where deleted_at is not null;

– result: 所有被标记为删除的 users

If you want to determine whether the values ​​of two columns are different, you can use is distinct from:

select * from users

where has_address is distinct from has_photo

– result: 地址(address)或照片(photo)两者只有其一的用户

　　not in and Null

Subquery (subselect) is a very convenient way to filter data. For example, if you want to query users who do not have any packages, you can write the following query:

select * from users 

where id not in (select user_id from packages)

But at this time, if the user_id of a row in the packages table is null, a problem arises: the return result is empty! To understand why this weird thing happens, we need to understand what the SQL compiler did. Below is a simpler example:

select * from users 

where id not in (1, 2, null)

This SQL statement will be converted to:

select * from users 

where id != 1 and id != 2 and id != null

We know that the result of id != null is an unknown value, null. And the result of the AND operation between any value and null is null, so it is equivalent to no other conditions. The reason for this result is that the logical value of null is not true. .

If the conditions are reversed, there will be no problem with the query results. Now we query users who have packages.

select * from users 

where id in (select user_id from packages)

​Similarly we can use a simple example:

select * from users

where id in (1, 2, null)

This SQL is converted to:

select * from users 

where id = 1 or id = 2 or id = null

Since the where clause is a series of or conditions, it doesn't matter if one of them results in null. Non-true values ​​do not affect the calculation results of other parts of the clause, and are equivalent to being ignored.

​Null and sorting

When sorting, the null value is considered the largest. This can cause a headache when sorting in descending order (descending), because the null value is ranked first.

The following query is to display user rankings based on scores, but it ranks users without scores to the top!

select name, points

from users

order by 2 desc;

– points 为 null 的记录排在所有记录之前!

There are two ways of thinking about solving this kind of problem. The simplest one is to use coalesce to eliminate the effect of null:

– 在输出时将 null 转换为 0 :

select name, coalesce(points, 0)

from users

order by 2 desc;

– 输出时保留 null, 但排序时转换为 0 :

select name, points

from users

order by coalesce(points, 0) desc;

There is another way that requires database support, which is to specify whether to put the null value first or last when sorting:

select name, coalesce(points, 0)

from users

order by 2 desc nulls last;

Of course, null can also be used to prevent errors from occurring, such as handling mathematical operation errors when the divisor is zero.

Divided by 0

Dividing by zero is a very egg-painfull error. SQL that was running fine yesterday suddenly went wrong when it was divided by 0. A common solution is to first use a case statement to determine whether the denominator is 0, and then perform the division operation.

select case when num_users = 0 then 0 

else total_sales/num_users end;

The way of the ase statement is actually ugly, and the denominator is reused. If it's a simple situation, it's okay. If the denominator is a very complex expression, then tragedy will come: it is difficult to read, difficult to maintain and modify, and there will be a lot of bugs if you are not careful.

At this time we can look at the benefits of null. Use nullif to make the denominator become null when it is 0. In this way, an error will no longer be reported. When num_users = 0, the return result becomes null.

select total_sales/nullif(num_users, 0);

nullif 是将其他值转为 null, 而Oracle的 nvl 是将 null 转换为其他值。

If you do not want null, but want to convert to 0 or other numbers, you can use the coalesce function based on the previous SQL:

select coalesce(total_sales/nullif(num_users, 0), 0);

null 再转换回0

Conclusion

Tony Hoare may regret his mistake, but at least the problem of null can be easily solved. So go practice your new ultimate move and stay away from the invalid pit (nullifying) dug by null!

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