Modifying dynamic column contents - sql

Question

I am trying to create a new column inside a dynamic column.
My table Templates just has two columns: ID, Structure (blob)
I run this query:
UPDATE `Templates` SET `Structure` = COLUMN_ADD(`Structure`, 'general', '') where `Templates`.`ID` = 1
Structure Result (Using COLUMN_JSON for display):
{"general":""}
Then I run this query:
UPDATE `Templates` SET `Structure` = COLUMN_ADD(COLUMN_GET(`Structure`, 'general' as CHAR), 'Inner', 'value') WHERE `Templates`.`ID` = 1
Structure Result:
{"Inner":"value"}
The result I want after both queries:
{"general": {"Inner":"value"}}
How can I get a column added to the dynamic "general" column instead of replacing the contents?

Answer 1

First, here is what happens with your query.
MariaDB [test]> CREATE TABLE Templates (ID INT, Structure BLOB);
Query OK, 0 rows affected (0.24 sec)
MariaDB [test]> INSERT INTO Templates VALUES (1, COLUMN_CREATE('general',''));
Query OK, 1 row affected (0.05 sec)
MariaDB [test]> SELECT COLUMN_JSON(Structure) FROM Templates;
+------------------------+
| COLUMN_JSON(Structure) |
+------------------------+
| {"general":""} |
+------------------------+
1 row in set (0.00 sec)
At this point you have in Structure one dynamic column with name general and empty string as a value.
Then you do this:
UPDATE `Templates`
SET `Structure` = COLUMN_ADD(
COLUMN_GET(`Structure`, 'general' as CHAR),
'Inner',
'value'
) ...
Your COLUMN_GET gets the value of general dynamic column, which is an empty string, and uses it as the first argument for COLUMN_ADD. It's a useless exercise, because if you want to run COLUMN_ADD on an empty string, you can just say so in the query or use COLUMN_CREATE; and if you want to actually add something to the existing value of the blob, you need to use the name of the blob.
So, COLUMN_ADD works on an empty string -- in other words, creates a clean new value for Structure, discarding everything it had -- and adds a dynamic column with name Inner and value value. That's why you are getting this:
MariaDB [test]> SELECT COLUMN_JSON(Structure) FROM Templates;
+------------------------+
| COLUMN_JSON(Structure) |
+------------------------+
| {"Inner":"value"} |
+------------------------+
1 row in set (0.00 sec)
Apparently, what you want to do instead is to set the value of general column to a new dynamic column.
You don't need to fetch general column for that, because COLUMN_ADD(x,y,z) will replace the value if a column y already exists in blob x. But you need to construct a new dynamic column for the new value of general.
So, what you should do is
UPDATE `Templates`
SET `Structure` = COLUMN_ADD(
`Structure`,
'general',
COLUMN_CREATE('Inner','value')
) ...
This accounts for a more general case when Structure also contains other columns, not only general, and you want to preserve them. If it's not the case, and you want to make sure the blob contains only general, then you can do
UPDATE `Templates`
SET `Structure` = COLUMN_CREATE(
'general',
COLUMN_CREATE('Inner','value')
)