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.
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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
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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.
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).
NULL means you do not have to provide a value for the field...
NOT NULL means you must provide a value for the fields.
For example, if you are building a table of registered users for a system, you might want to make sure the user-id is always populated with a value (i.e. NOT NULL), but the optional spouses name field, can be left empty (NULL)
I would suggest
- Use NOT NULL on every field if you can
- Use NULL if there is a sensible reason it can be null
Having fields which don't have a meaningful meaning for NULL nullable is likely to introduce bugs, when nulls enter them by accident. Using NOT NULL prevents this.
The commonest reason for NULL fields is that you have a foreign key field which is options, i.e. not always linked, for a "zero or one" relationship.
If you find you have a table with lots of columns many of which can be NULL, that starts sounding like an antipattern, consider whether vertical partitioning makes more sense in your application context :)
There is another useful use for NULL - making all the columns in an index NULL will stop an index record being created for that row, which optimises indexes; you may want to index only a very small subset of rows (e.g. for an "active" flag set on only 1% or something) - making an index which starts with a column which is usually NULL saves space and optimises that index.
Disclaimer: Because the syntax for filtering in QGIS works with SQL, I'm assuming here that SQL rules apply. I'm not completely sure if that's entirely correct, but it seems logical and it does explain the behavior.
The filter works with SQL, that's why you have to look there for an answer.
In short, when using a logical operator in combination with null, the result is always null. But in order to test for null, SQL comes with the IS (NOT) comparison functionality, which allows to use it for your intended filtering.
Check @Bohemian's answer on StackOverflow for a more in-depth discussion.
NULL is not a value, therefore it cannot equal = or not equal != anything. It is not the same as zero 0 which is a value.
A NULL indicates that no value has been recorded in the cell you are looking at. To check if a value exists you ask if the cell IS NULL or if it IS NOT NULL
IS NULLchecks to see if the cell is emptyIS NOT NULLchecks to see if the cell is not empty
If you have some records where a values are One, Two, Three and the rest NULL and you want to find everything that isn't Two you would need to use something like
value != 'Two' OR value IS NULL
as NULL values do not get returned in an equals/not equals query. If you used just value != 'Three' the result would exclude all NULL records as NULL isn't a value that can equal or not equal.