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Why does NULL = NULL evaluate to false in SQL server
If you generate a query to insert the data in table "MyTab" for column --- Age, Sex, DOB, ID
INSERT INTO MyTab
VALUES (22, '', '', 4)
What'll be the value in column Sex & DOB ? Is it NULL ?
If value is NULL then ---
SELECT * FROM MyTab
WHERE Sex=NULL
above query gives output ---- no rows selected --- why ??
if value is not NULL then ---
SELECT * FROM Mytab
WHERE Sex IS NULL
above query gives the output ---- how ??
599
You can insert NULL values into columns with the UNIQUE constraint because NULL is the absence of a value, so it is never equal to other NULL values and not considered a duplicate value. This means that it's possible to insert rows that appear to be duplicates if one of the values is NULL .
What is a NULL Value? A field with a NULL value is a field with no value. If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field. Then, the field will be saved with a NULL value.
A NOT NULL constraint is a rule that prevents null values from being entered into one or more columns within a table. A unique constraint (also referred to as a unique key constraint) is a rule that forbids duplicate values in one or more columns within a table.
Discussion: If you want the query to return only unique rows, use the keyword DISTINCT after SELECT . DISTINCT can be used to fetch unique rows from one or more columns. You need to list the columns after the DISTINCT keyword.
NULL is a special value in SQL denoting the absence of data. As such, you cannot do queries like:
SELECT fields FROM table WHERE column = NULL
NULL cannot be compared to anything, including NULL itself. Instead, you'd need:
SELECT fields FROM table WHERE column IS NULL
However in your case you are really inserting an empty value in Sex and DOB.
And empty value is not NULL. You'd have to query for:
SELECT fields FROM table WHERE column = ''
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