Sure! Here's a clear explanation of those general functions related to handling NULLs and conditional logic in SQL:

General Functions: NVL, NVL2, DECODE, COALESCE, NULLIF, LNNVL, NANVL

1. NVL(expr1, expr2)

• Replaces NULL value expr1 with expr2.

• Returns expr1 if it is not NULL; otherwise, returns expr2.

SELECT NVL(NULL, 'Default') FROM dual;   -- Returns 'Default'SELECT NVL('Hello', 'Default') FROM dual; -- Returns 'Hello'

Common in Oracle.

2. NVL2(expr1, expr2, expr3)

• If expr1 is not NULL, returns expr2;

• Otherwise, returns expr3.

SELECT NVL2('Test', 'Not Null', 'Is Null') FROM dual;  -- Returns 'Not Null'SELECT NVL2(NULL, 'Not Null', 'Is Null') FROM dual;    -- Returns 'Is Null'

3. DECODE(expr, search, result [, search, result ...] [, default])

• Compares expr to each search value; returns corresponding result if matched; otherwise, returns default (or NULL if default is not given).

• Works like a simple IF-THEN-ELSE or CASE statement.

SELECT DECODE(2, 1, 'One', 2, 'Two', 'Other') FROM dual;  -- Returns 'Two'SELECT DECODE(3, 1, 'One', 2, 'Two', 'Other') FROM dual;  -- Returns 'Other'

4. COALESCE(expr1, expr2, ..., exprN)

• Returns the first non-NULL expression in the list.

• More flexible than NVL because it can take multiple arguments.

SELECT COALESCE(NULL, NULL, 'Third', 'Fourth') FROM dual;  -- Returns 'Third'

5. NULLIF(expr1, expr2)

• Returns NULL if expr1 = expr2; otherwise, returns expr1.

• Useful to avoid division by zero or similar conditions.

SELECT NULLIF(10, 10) FROM dual;  -- Returns NULLSELECT NULLIF(10, 5) FROM dual;   -- Returns 10

6. LNNVL(condition)

• Logical Negation of Null Value Logic.

• Returns TRUE if condition is FALSE or UNKNOWN (NULL).

• Returns FALSE if condition is TRUE.

• Mainly used in WHERE clauses to handle three-valued logic with NULLs.

SELECT * FROM employees WHERE LNNVL(salary > 5000);-- Returns rows where salary <= 5000 or salary IS NULL

7. NANVL(expr1, expr2)

• Returns expr2 if expr1 is NaN (Not a Number); otherwise returns expr1.

• Used to handle NaN values in floating-point operations.

SELECT NANVL(0/0, 0) FROM dual;  -- Returns 0 (since 0/0 is NaN)

Summary Table:

If you want examples with your data or specific SQL dialect (Oracle, PostgreSQL, etc.), just ask!