PostgreSQL - COALESCE

In PostgreSQL, the COALESCE function is a valuable tool that allows you to return the first non-NULL value from a list of expressions. If all the expressions evaluate to NULL, then the COALESCE function will return NULL.

Syntax:

COALESCE(expression1, expression2, ..., expressionN)

The function will evaluate the expressions from left to right and return the first non-NULL value it encounters.

Examples:

Simple Use Case:

Suppose you have a table called products with columns discount_price and original_price. If discount_price is NULL for some products, you might want to display original_price instead.

SELECT product_name,
       COALESCE(discount_price, original_price) AS sale_price
FROM products;

Multiple Expressions:

The power of COALESCE becomes clear when you have multiple fallback values.

SELECT COALESCE(NULL, NULL, 'third value', 'fourth value');

This will return 'third value' because it's the first non-NULL value in the list.

Combined with Other Functions:

You can use COALESCE with other functions to handle NULL values gracefully. For instance, let's say you want to get the length of a string from a column, but it might be NULL:

SELECT COALESCE(LENGTH(column_name), 0) AS string_length
FROM table_name;

If column_name is NULL, the LENGTH function would return NULL, but the COALESCE function ensures that 0 is returned instead.

Points to Note:

COALESCE is essentially a shorthand for a series of CASE statements. For example, COALESCE(a, b, c) is equivalent to:

CASE 
    WHEN a IS NOT NULL THEN a
    WHEN b IS NOT NULL THEN b
    ELSE c
END

COALESCE can be especially useful when dealing with outer joins, where the joined table might not have a matching row, resulting in NULL values.
Always ensure that the data types of the expressions provided to COALESCE are compatible. If they are not, you might encounter errors or unexpected results.

In summary, the COALESCE function in PostgreSQL is a handy utility for handling NULL values, allowing you to specify fallback values and ensure more predictable query results.

Using COALESCE for handling NULL values in PostgreSQL:
- Description: COALESCE is used to return the first non-null expression from a list. It's handy for handling NULL values.
- Code:
```
SELECT COALESCE(my_column, 'Default Value') AS my_result FROM my_table;
```
COALESCE vs. NULLIF in PostgreSQL:
- Description: While COALESCE returns the first non-null expression, NULLIF compares two expressions and returns null if they are equal.
- Code (COALESCE):
```
SELECT COALESCE(my_column, 'Default Value') AS my_result FROM my_table;
```
- Code (NULLIF):
```
SELECT NULLIF(my_column, 'Invalid Value') AS my_result FROM my_table;
```
COALESCE with multiple arguments in PostgreSQL:
- Description: COALESCE can take multiple arguments. It returns the first non-null expression from the list.
- Code:
```
SELECT COALESCE(column1, column2, column3, 'Fallback Value') AS my_result FROM my_table;
```
Replacing NULL values with default values using COALESCE in PostgreSQL:
- Description: COALESCE is often used to replace NULL values with default values.
- Code:
```
SELECT COALESCE(my_column, 'Default Value') AS my_result FROM my_table;
```
Using COALESCE in SELECT statements in PostgreSQL:
- Description: COALESCE is frequently used in SELECT statements to handle NULL values gracefully.
- Code:
```
SELECT column1, COALESCE(column2, 'N/A') AS formatted_column FROM my_table;
```

COALESCE vs CASE in PostgreSQL:

Description: Both COALESCE and CASE can be used to handle NULL values. COALESCE is concise for simple cases, while CASE provides more flexibility.

Code (COALESCE):

SELECT COALESCE(my_column, 'Default Value') AS my_result FROM my_table;

Code (CASE):

SELECT
    CASE
        WHEN my_column IS NULL THEN 'Default Value'
        ELSE my_column
    END AS my_result
FROM my_table;

Nested COALESCE functions in PostgreSQL:
- Description: COALESCE functions can be nested to handle multiple levels of potential NULL values.
- Code:
```
SELECT COALESCE(COALESCE(column1, column2), 'Fallback Value') AS my_result FROM my_table;
```