7 ms·

> Also, understand how your DB handles nulls. Also in regards to indexing. The DBs I've used have not indexed nulls, so a "WHERE col IS NULL" is inefficient ev

> Also, understand how your DB handles nulls. Also in regards to indexing. The DBs I've used have not indexed nulls, so a "WHERE col IS NULL" is inefficient even though "col" is indexed. If that is the case and you really need it, have a computed column with a char(1) or bit indicating if "col" is NULL or not, and index that.

SoftTalker 1y ago
NULL should generally never be used to "mean" anything. If your business rules say that "not applicable" or "no entry" is a value, store a value that indicates that, don't use NULL.
- rplnt 1y ago
  Interesting, I don't think I've seen that while NULLs are very common. I guess you would handle it in the application and not in the query, right?
  - SoftTalker 1y ago
    I've seen it too, very often. But it's good if you can just keep NULL meaning NULL (i.e. "the absence of any value"), because otherwise you will eventually be surprised by behavior.
- crazygringo 1y ago
  Not sure what you mean. If you have a table of customers and someone of them don't have addresses, it's standard to leave the address fields NULL. If some of them don't belong to a company, it's standard to leave the company_id field NULL. This is literally what NULL is for. It's a special value precisely because missing data or a N/A field is so common. If you're suggesting mandatory additional has_address and has_customer_id fields, I would disagree. You'd be reinventing a database tool that already exists precisely for that purpose.
  - MaxBarraclough 1y ago
    > This is literally what NULL is for. It's a special value precisely because missing data or a N/A field is so common. Kinda. You need null for outer joins, but you could have a relational DBMS that prohibits nullable columns in tables. Christopher Date thought that in properly normalised designs, tables should never use nullable columns. Codd disagreed. [0] > If you're suggesting mandatory additional has_address and has_customer_id fields, I would disagree. You'd be reinventing a database tool that already exists precisely for that purpose. The way to do it without using a nullable column is to introduce another table for the 'optional' data, and use a left outer join. [0] https://en.wikipedia.org/wiki/First_normal_form#Christopher_J._Date's_definition_of_1NF https://en.wikipedia.org/wiki/First_normal_form#Christopher_...
    - crazygringo 1y ago
      > The way to do it without using a nullable column I mean, you could, but having separate tables for every optional field would be an organizational and usability nightmare. Queries would be longer and slower for no good reason. Not to mention a gigantic waste of space with all those repeated primary keys and their indexes. And you could have databases that prohibited NULL values, but we mostly don't, because they're so useful.
      - naasking 1y ago
        > but having separate tables for every optional field would be an organizational and usability nightmare I think this indicates that declaring and managing state is too onerous in SQL.
        crazygringo 1y ago
        Or maybe NULLs are actually a great solution here, and it's fine. The idea that having a separate table for every optional field is too unworkable isn't an issue with SQL. It's a fundamentally overcomplicated idea. It's like a programming language where every variable that could be null had to be put in its own file. It would be terrible design.
- Breza 1y ago
  I've found it's easier just to keep things NULL in many cases. Sure, if you have a student database with the column favorite_sport, maybe it makes sense to use the value "Not a sports fan." But think about an email list with the column unsubscribe_date. For active recipients, you'll want that field to be NULL.

SoftTalker 1y ago

NULL should generally never be used to "mean" anything. If your business rules say that "not applicable" or "no entry" is a value, store a value that indicates that, don't use NULL.

rplnt 1y ago
Interesting, I don't think I've seen that while NULLs are very common. I guess you would handle it in the application and not in the query, right?
- SoftTalker 1y ago
  I've seen it too, very often. But it's good if you can just keep NULL meaning NULL (i.e. "the absence of any value"), because otherwise you will eventually be surprised by behavior.
crazygringo 1y ago
Not sure what you mean. If you have a table of customers and someone of them don't have addresses, it's standard to leave the address fields NULL. If some of them don't belong to a company, it's standard to leave the company_id field NULL. This is literally what NULL is for. It's a special value precisely because missing data or a N/A field is so common. If you're suggesting mandatory additional has_address and has_customer_id fields, I would disagree. You'd be reinventing a database tool that already exists precisely for that purpose.
- MaxBarraclough 1y ago
  > This is literally what NULL is for. It's a special value precisely because missing data or a N/A field is so common. Kinda. You need null for outer joins, but you could have a relational DBMS that prohibits nullable columns in tables. Christopher Date thought that in properly normalised designs, tables should never use nullable columns. Codd disagreed. [0] > If you're suggesting mandatory additional has_address and has_customer_id fields, I would disagree. You'd be reinventing a database tool that already exists precisely for that purpose. The way to do it without using a nullable column is to introduce another table for the 'optional' data, and use a left outer join. [0] https://en.wikipedia.org/wiki/First_normal_form#Christopher_J._Date's_definition_of_1NF https://en.wikipedia.org/wiki/First_normal_form#Christopher_...
  - crazygringo 1y ago
    > The way to do it without using a nullable column I mean, you could, but having separate tables for every optional field would be an organizational and usability nightmare. Queries would be longer and slower for no good reason. Not to mention a gigantic waste of space with all those repeated primary keys and their indexes. And you could have databases that prohibited NULL values, but we mostly don't, because they're so useful.
    - naasking 1y ago
      > but having separate tables for every optional field would be an organizational and usability nightmare I think this indicates that declaring and managing state is too onerous in SQL.
      - crazygringo 1y ago
        Or maybe NULLs are actually a great solution here, and it's fine. The idea that having a separate table for every optional field is too unworkable isn't an issue with SQL. It's a fundamentally overcomplicated idea. It's like a programming language where every variable that could be null had to be put in its own file. It would be terrible design.
Breza 1y ago
I've found it's easier just to keep things NULL in many cases. Sure, if you have a student database with the column favorite_sport, maybe it makes sense to use the value "Not a sports fan." But think about an email list with the column unsubscribe_date. For active recipients, you'll want that field to be NULL.