sql - NULL vs NOT NULL - Stack Overflow
sql - TSQL - When to use 'not null' - Stack Overflow
What is the meaning of NULL and NOT NULL in MySQL database - Stack Overflow
Need some knowledge on NULL and NOT NULL
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Where and why exactly a null is used?
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What is exactly null and not null? To my understanding Not null we use when its mandatory to insert some value in that field, also when we give check constraint so by default the column will be not null right?
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By adding new column through alter method default values are null, so how would I be able to insert values in it and is it right to give not null constraint to that new column while adding through alter method, basically when null and when not null to be used?...
god this is so confusing please help me, ik im asking alot but im really confused
If you need to represent unknown data in a column, you make it nullable. If you will always have data in the column, it's better to make it not nullable, as
- Dealing with nulls can be annoying and counterintuitive
- It saves a bit of space
- On some database systems, null values are not indexed.
When a field is set to NOT NULL, it cannot be empty. Which means you have to specify a value for that field when inserting a record.
Depends on how you want your application to behave.
- First, there will never ever ever be a possible row where this value does NOT contain meaningful data, then use NOT NULL. That is, this value will always be meaningful and represent something.
- Do you want the value to always be filled out by the user or programmer in some form or fashion? Use NOT NULL without a DEFAULT
- Do you want it to be optional to users and programmers? Use NOT NULL with a DEFAULT
I think you've got 2 questions there:
Should you mark fields as not null?
Yes, assuming that you never intend a valid row to have a null value in that field. Think of "not null" as the easiest type of constraint you can put on a field. Constraints in a database help ensure the data is kept consistent by meeting expectations.
Should not null fields have defaults?
Only when there is an obvious default. For example the Paid field of an invoices table might have a default of 0 (false). In general it works the other way around - if a field has a default value, then it should probably also be not null.
Don't create defaults just for the sake of defaults - if a field should not be null, but there isn't a universal default, then leave it be. That ensures that any INSERT statements must provide a value for that field.
NULL is used to represent "no value" and allow that to be distinguished between 1 and 0, true or false, or an empty string versus a string with content. It's similar to nil in Ruby, null in JavaScript or NULL in PHP.
If you define a column as NOT NULL then it won't allow an INSERT without a value for that column being specified. If you have a DEFAULT then this will be applied automatically. If you use an ORM it may fill it in for you with a safe, minimal default.
Columns that can be NULL require an almost insignificant amount of additional storage per row, one bit, to hold the NULL or NOT NULL flag.
Remember that NULL in MySQL is unusual in that it is not greater than, less than, or equal to any other value. This is why IS NULL and IS NOT NULL are required for logical comparisons.
The checkbox probably sets the default value to null. There may also be a box that specifies whether or not the field accepts null values. Null basically means Empty or Nothing (in VB).