Absolute value in DBMS | database management system

 

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Null value

In the database, a common issue is which value you use to represent the missing values. In SQL, this is resolved with null. It is used to denote missing or unknown values. NULL is used to represent these values.

NULL is not really a specific value as much as it is an indicator. Completely different from absolute zero or empty is not the same. The values ​​are zero (0) and empty “”.

  • Used to represent a null value The following different interpretation value is unknown (Value exists but not known)
  • Value not available (Present, but intentionally hidden)
  • Is not applicable (Undefined for that line)

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When it is not possible to code your data specifically using “N / A”, you can use the special keyword NULL to denote a missing value. In the general sense NULL has no value. For example, no two NULLs are equal to each other. NULL = NULL is FALSE!

You can use the IS NULL and IS NOT NULL comparisons to test for absolute values.

Is the NULL operator

All operations on the null values ​​present in the table must be performed using this ‘null’ operator .We cannot compare the null value using the assignment operator

Not full barrier

  • Not all constraints stop a column to contain null values
  • Once Null does not apply to a particular column, you cannot enter a null value in that column And is restricted to only maintain some fair value other than zero
  • a The null constraint cannot be applied at the table level

All total functions, except COUNT, ignore NULL values. If we query to find the maximum and minimum of the average digit, it will ignore the null value and give the result.

Any arithmetic operation with a NULL column will result in NULL values. If we consider the example whether one of the students has not yet attained his average mark. If we try to add the avg_mark column, it will result in a NULL value.

Select MAX (AVG_MARK), Min (AVG_MARK) From student;

Select COUNT (AVG_MARK) From student;

Any comparison operator used with a NULL value will itself return a null value.

Choose * From student Where on STD_ID = 100 And AVG_MARK> 50;

important point

  • NULL is a value space holder for the optional table field.
  • MySQL treats the NULL value differently from other data types. The NULL value, when used in a condition, evaluates to a false Boolean value.
  • The NOT logical operator is used to test for boolean values ​​and evaluate if the boolean value is true and false if the boolean value is true.
  • The NOT NULL clause is used to eliminate the NULL value from the result set
  • Performing arithmetic operations on NULL values ​​always returns NULL results.
  • Comparison operators such as [, =, etc.] Can not be used to compare NULL values.

FAQ’s

How to check if a value is zero?

SQL allows queries that check whether an attribute value is NULL. Instead of using = or NULL, use SQL to compare an attribute value is And Is not. This is because SQL considers each NULL value to be different from every other NULL value, so the comparison of equality is not appropriate.

Real life example?

If students are numeric columns in the table and the result is yet to come. In that case, all values ​​in the numeral column are not zero. It will be defined as zero.


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