Get First Non-null Value From Selected Rows For Every Column - mysql

Question

I am trying to create a query that can get First Non-null value for selected columns from the table.
I cant fire multiple queries and union it per every column as I have so many columns. I tried to create a query using answers form some SO questions. but it doesn't work for me.
Example Table
| orders| id | default_address |
|-------|------|-----------------|
| 1 | 1 | null |
| 2 | null | null |
| 3 | 2 | 3 |
Expected Result
| id | default_address |
|----|-----------------|
| 1 | 3 |
Another Example Table
| orders| id | default_address |
|-------|------|-----------------|
| 1 | 1 | null |
| 2 | null | 5 |
| 3 | 2 | 3 |
Expected Result
| id | default_address |
|----|-----------------|
| 1 | 5 |
What i tried is here
http://sqlfiddle.com/#!9/84a5c/1
http://sqlfiddle.com/#!9/574481/2

Answer 1

Here is one way to do this using analytic functions, assuming you are using MySQL 8+:
WITH cte AS (
SELECT *,
COUNT(id) OVER (ORDER BY orders) cnt_id,
COUNT(default_address) OVER (ORDER BY orders) cnt_addr
FROM yourTable
)
SELECT
MAX(CASE WHEN cnt_id = 1 THEN id END) AS id,
MAX(CASE WHEN cnt_addr = 1 THEN default_address END) AS default_address
FROM cte;
This assumes that there actually exist a third column orders which generates the ordering shown in your sample data.
This trick works because the COUNT() function by default only counts non NULL values. So, used a window function with the ordering given by the orders column, it would only equal 1 at the first non NULL value.
For earlier MySQL versions:
SELECT
MAX(id) AS id,
MAX(default_address) AS default_address
FROM
(
SELECT
CASE WHEN (SELECT COUNT(t2.id) FROM yourTable t2
WHERE t2.orders <= t1.orders) = 1 THEN id END AS id,
CASE WHEN (SELECT COUNT(t2.default_address) FROM yourTable t2
WHERE t2.orders <= t1.orders) = 1 THEN default_address END AS default_address
FROM yourTable t1
) t;