I need to do retrofit query using update or merge - sql

Question

I have two tables A and B. In A, I have a column called fetch_year. I need to consider table B from these two columns
primary_date
secondary_date
These columns have JSON values like {"lock":"true","date":"01/01/1990"}
So from this, I need to get the date and I need to extract the year and should save it in table A column called fetch_year. Will always consider primary_date first then secondary_date(if primary_date is null)
The final result should be 1990 in the fetch_year column
Table A is empty as of now( only one column with cal_id)
cal_id fetch_year
1 null
n null
Table B
|B_id|Cal_id | primary_date | secondary_date |
|----|-------|-----------------------------------|------------------------|
|11 | 1 |{"lock":"true","date":"01/01/1990"}|Null|
|12 | 2 | Null | {"lock":"true","date":"01/01/1980"} |
|13 | 3 | Null | Null |
|14 | 4 | {"lock":"true","date":"01/01/1995"} |{"lock":"true","date":"01/01/1997"} |
In table B
So I have n number of records in both the tables
I need results like this in A table
Cal_id fetch_year.
1 1990
2 1980
3 Null
4 1995
n n-values
In cal_id =4 in this case we have value in both columns so we are considering primary_date not secondary_date
Please help me with this problem

Answer 1

You could make use of either JSON_VALUE or OPENJSON here to extract the date from your JSON blobs.
I tend to prefer OPENJSON because it allows you to extract multiple values simultaneously and they don't have to be at the same level in a nested JSON structure. With the "squirelly" dates in your example data, though, you may prefer the JSON_VALUE version with TRY_CONVERT so that you have more control over date deserialization.
--Data setup
create table dbo.A (
Cal_id int,
fetch_year int
);
create table dbo.B (
B_id int not null identity(11,1),
Cal_id int,
primary_date nvarchar(max),
secondary_date nvarchar(max)
);
insert dbo.A (Cal_id, fetch_year)
values
(1, null),
(2, null),
(3, null),
(4, null);
insert dbo.B (Cal_id, primary_date, secondary_date)
values
(1, N'{"lock":"true","date":"01/01/1990"}', null),
(2, null, N'{"lock":"true","date":"01/01/1980"}'),
(3, null, null),
(4, N'{"lock":"true","date":"01/01/1995"}', N'{"lock":"true","date":"01/01/1997"}');
--JSON_VALUE example
update Table_A
set fetch_year = year(coalesce(
-- REF: CAST and CONVERT / Date and time styles
-- https://learn.microsoft.com/en-us/sql/t-sql/functions/cast-and-convert-transact-sql#date-and-time-styles
try_convert(date, json_value(primary_date, '$.date'), 101), --mm/dd/yyyy
try_convert(date, json_value(secondary_date, '$.date'), 101) --mm/dd/yyyy
))
from dbo.A Table_A
join dbo.B Table_B on Table_B.Cal_id = Table_A.Cal_id
--OPENJSON example
update Table_A
set fetch_year = year(coalesce(
Primary_JSON.date,
Secondary_JSON.date
))
from dbo.A Table_A
join dbo.B Table_B on Table_B.Cal_id = Table_A.Cal_id
outer apply openjson(Table_B.primary_date) with ([date] date) Primary_JSON
outer apply openjson(Table_B.secondary_date) with ([date] date) Secondary_JSON;