How to group columns into separate rows in SQL - mysql

Question

I don't have a profound SQL background, recently encountered a problem with SQL that seems hard to do with JUST SQL.
I have a table
```
IMEI | DATE | A_1 | A_2 | A_3 | B_1 | B_2 | B_3
2132 | 09/21| 2 | 4 | 4 | 5 | 2 | 4
4535 | 09/22| 2 | 2 | 4 | 5 | 2 | 3
9023 | 09/21| 2 | 1 | 5 | 7 | 2 | 2
```
How can I group value of A_1, A_2 etc in a way so I can achieve the this table. Basically, I would like to group certain columns in my table, and put them into different rows.
IMEI | DATE | MODULE | val_1 | val_2 | val_3
2132 | 09/21| A | 2 | 4 | 4
2132 | 09/21| B | 5 | 2 | 4
...
The goal is to have value under namespace A, B etc for a row to be separated into different rows in the new Table.
Also, any suggestions on where can I improve my SQL. any books I should keep as reference or any other resources I should use?
Thanks!

Answer 1

You can do it with UNION:
SELECT IMEI, DATE, 'A' AS MODULE, A_1 AS val_1, A_2 AS val_2, A_3 AS val_3
FROM myTable
UNION ALL
SELECT IMEI, DATE, 'B', B_1, B_2, B_3
FROM myTable
See it on sqlfiddle.
But really, you should store your data in the form created by the above query, and then use a JOIN to create the original format as/when desired.

Answer 2

I love playing with data and questions like this!
Below can be considered as over-engineering but I think it is still an option when you don't know your columns names in advance but have a pattern you described or it can be useful just for learning as looks like you are looking for improving your SQL (based on tag for this question I assume you meant BigQuery SQL)
#standardSQL
WITH parsed AS (
SELECT IMEI, DATE,
REGEXP_REPLACE(SPLIT(row, ':')[OFFSET(0)], r'^"|"$', '') key,
REGEXP_REPLACE(SPLIT(row, ':')[OFFSET(1)], r'^"|"$', '') value
FROM `yourTable` t,
UNNEST(SPLIT(REGEXP_REPLACE(to_json_string(t), r'[{}]', ''))) row
),
grouped AS (
SELECT
IMEI, DATE,
REGEXP_EXTRACT(key, r'(.*)_') MODULE,
ARRAY_AGG(value ORDER BY CAST(REGEXP_EXTRACT(key, r'_(.*)') AS INT64)) AS vals
FROM parsed
WHERE key NOT IN ('IMEI', 'DATE')
GROUP BY IMEI, DATE, MODULE
)
SELECT IMEI, DATE, MODULE,
vals[SAFE_OFFSET(0)] AS val_1,
vals[SAFE_OFFSET(1)] AS val_2,
vals[SAFE_OFFSET(2)] AS val_3,
vals[SAFE_OFFSET(3)] AS val_4
FROM grouped
-- ORDER BY IMEI, DATE, MODULE
You can test / play with dummy data from your question
#standardSQL
WITH `yourTable` AS (
SELECT 2132 IMEI, '09/21' DATE, 2 A_1, 4 A_2, 4 A_3, 5 B_1, 2 B_2, 4 B_3 UNION ALL
SELECT 4535, '09/22', 2, 2 ,4, 5, 2, 3 UNION ALL
SELECT 9023, '09/21', 2, 1 ,5, 7, 2, 2
),
parsed AS (
SELECT IMEI, DATE,
REGEXP_REPLACE(SPLIT(row, ':')[OFFSET(0)], r'^"|"$', '') key,
REGEXP_REPLACE(SPLIT(row, ':')[OFFSET(1)], r'^"|"$', '') value
FROM `yourTable` t,
UNNEST(SPLIT(REGEXP_REPLACE(to_json_string(t), r'[{}]', ''))) row
),
grouped AS (
SELECT
IMEI, DATE,
REGEXP_EXTRACT(key, r'(.*)_') MODULE,
ARRAY_AGG(value ORDER BY CAST(REGEXP_EXTRACT(key, r'_(.*)') AS INT64)) AS vals
FROM parsed
WHERE key NOT IN ('IMEI', 'DATE')
GROUP BY IMEI, DATE, MODULE
)
SELECT IMEI, DATE, MODULE,
vals[SAFE_OFFSET(0)] AS val_1,
vals[SAFE_OFFSET(1)] AS val_2,
vals[SAFE_OFFSET(2)] AS val_3,
vals[SAFE_OFFSET(3)] AS val_4
FROM grouped
ORDER BY IMEI, DATE, MODULE
Output will be as below
Row IMEI DATE MODULE val_1 val_2 val_3 val_4
1 2132 09/21 A 2 4 4 null
2 2132 09/21 B 5 2 4 null
3 4535 09/22 A 2 2 4 null
4 4535 09/22 B 5 2 3 null
5 9023 09/21 A 2 1 5 null
6 9023 09/21 B 7 2 2 null