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ISNULL 
ISNULL 

ISNULL 

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return 1 if the argument is NULL, otherwise, return zero.

🧾 MySQL ISNULL() Function – Complete Study Note

📘 1. Purpose of ISNULL() in MySQL

In MySQL, the ISNULL() function is used to check whether a value is NULL. It returns 1 (true) if the expression is NULL, and 0 (false) if it is not.

✅ Why is this important?

  • NULL values represent missing, undefined, or unknown data.
  • You often need to filter out or detect rows where data is missing.
  • ISNULL() is a handy tool for conditionally identifying such values.

📌 2. Syntax of ISNULL()

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ISNULL(expression)
  • expression: Any column, value, or result of an operation.
  • Returns:
    • 1 if the expression is NULL
    • 0 if the expression is not NULL

🧪 3. Basic Examples and Explanation

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SELECT ISNULL(NULL);       -- Output: 1
SELECT ISNULL(5);          -- Output: 0
SELECT ISNULL(5 + NULL);   -- Output: 1
SELECT ISNULL(1 / 0);      -- Output: 1 (division by zero treated as NULL)

📝 Explanation:

  • NULL is obviously NULL → returns 1.
  • 5 is a number → not NULL → returns 0.
  • 5 + NULL results in NULL → returns 1.
  • 1 / 0 normally raises a division error, but MySQL returns NULL in this context → returns 1.

🔄 4. Difference Between ISNULL() and IS NULL Operator

There are two ways to check for NULL in MySQL:

Method
Description
ISNULL(expr)
Function that returns 1 or 0
expr IS NULL
SQL operator that returns a boolean result

✅ Example using IS NULL:

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SELECT * FROM customers WHERE state IS NULL;

✅ Equivalent using ISNULL():

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SELECT * FROM customers WHERE ISNULL(state);

✅ Both return the same result, but the IS NULL syntax is more commonly used in filters.

🏁 5. Special Use Case with DATE and '0000-00-00'

In MySQL, a date field declared as NOT NULL can still contain a "zero" date: '0000-00-00'.

🧪 Example:

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CREATE TABLE special_isnull (
    start_date DATE NOT NULL
);

INSERT INTO special_isnull(start_date)
VALUES ('2000-01-01'),
       ('0000-00-00');

Although start_date is NOT NULL, MySQL allows the special "zero date" '0000-00-00' for compatibility with ODBC.

🔍 Querying This Table:

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SELECT *
FROM special_isnull
WHERE ISNULL(start_date);

Expected: No rows (because NOT NULL constraint)

Actual: Returns one row where start_date = '0000-00-00'

📝 Explanation:

  • ISNULL(start_date) returns 1 for '0000-00-00'
  • MySQL treats '0000-00-00' as a special case in some settings (especially for ODBC compliance)

🔄 6. Negating ISNULL() and IS NULL

Operation
Syntax Example
Meaning
Check if NULL
ISNULL(date)
Is date field NULL?
Not NULL (operator)
date IS NOT NULL
Opposite of IS NULL
Not NULL (function)
!ISNULL(date)
Logical NOT for function version

🚧 7. MySQL vs. SQL Server: ISNULL() Confusion

⚠️ Important distinction:

  • In SQL Server, ISNULL(expr1, expr2) is a substitute function, equivalent to MySQL’s IFNULL().
  • In MySQL, ISNULL() is a boolean function used to check for NULL.

🧠 So if you come from SQL Server:

In SQL Server
Use in MySQL
ISNULL(col, 'default')
IFNULL(col, 'default')

✅ 8. Summary of Differences

Feature
ISNULL() (MySQL)
IS NULL Operator
IFNULL() (MySQL)
Purpose
Checks if value is NULL
Checks if column is NULL
Replaces NULL with a default
Return Value
1 or 0
TRUE or FALSE
Returns value or default
Accepts Expressions?
✅ Yes
✅ Yes
✅ Yes (but needs 2 args)
Alternative in SQL Server
❌ Different behavior
✅ Standard
✅ Same as SQL Server’s ISNULL()

🧠 Final Takeaways

  • Use ISNULL(expr) when you want a binary result: 1 for NULL, 0 for non-NULL.
  • Use IS NULL or IS NOT NULL when writing conditions in WHERE, CASE, or JOIN clauses.
  • Do not confuse ISNULL() in MySQL with SQL Server’s version—use IFNULL() for substitution logic instead.
  • Remember that MySQL treats '0000-00-00' as a special default date, which may cause ISNULL() to behave unexpectedly.