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SQL NULL Functions: Main Tips
- The function ISNULL() is needed in order to define the way we use NULL.
- There are other three functions which will produce a similar result ( COALESCE(), IFNULL(), NVL() ).
Syntax of SQL NULL
Below you can see the table "Products":
| ID | Name | Supplier_ID | Category_ID | Unit | Price | Unit_In_Stock | Unit_On_Order |
|---|---|---|---|---|---|---|---|
| 1 | Cucumber | 3 | 3 | 5 crates | 19 | 17 | 16 |
| 2 | Tomato | 3 | 3 | 6 crates | 20 | 24 | Null |
| 3 | Cheese | 3 | 4 | 5 kg boxes | 11 | 11 | 6 |
| 4 | Milk | 4 | 4 | 5 L bottles | 22 | 56 | 12 |
| 5 | Bread | 4 | 4 | 36 boxes | 26 | 26 | 1 |
Have in mind that UnitOnOrder has NULL values.
In the code example below we can see that the result will be NULL if the columns "Unit_On_Order" will contain NULL values:
Example
SELECT Name, Price * (Unit_In_Stock + Unit_On_Order)
FROM ProductsThe examples below show that if the column "UnitOnOrder" is a value NULL the enumeration will proceed as usual. The function ISNULL() brings back a zero when NULL is the value:
MS Access
Example
SELECT Name, Price * (Unit_In_Stock + IIF(ISNULL(Unit_On_Order), 0, Unit_On_Order))
FROM Products;
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SQL Server
Example
SELECT Name, Price * (Unit_In_Stock + ISNULL(Unit_On_Order, 0))
FROM ProductsOracle
Here we start using other functions rather than ISNULL() because of the database-system compatibility. In oracle we use the function NVL():
Example
SELECT Name, Price * (Unit_In_Stock + NVL(Unit_On_Order, 0))
FROM ProductsMySQL
Just like in oracle, MySQL requires a different function to work. There are two that work:
First, we have the function IFNULL():
Example
SELECT Name, Price * (Unit_In_Stock + IFNULL(Unit_On_Order, 0))
FROM ProductsThe second function is COALESCE():
Example
SELECT Name, Price * (Unit_In_Stock + COALESCE(Unit_On_Order, 0))
FROM Products
