How to handle T-SQL missing columns when using OPENJSON function? - sql-server

Question

Consider the three T-SQL scenarios below:
-- scenario 1: SSN is specified with a value
DECLARE #Json_Array nvarchar(max) = '[{"Employee_ID":123,"Employee_Name":"John Doe","SSN":123456789}]';
SELECT *
FROM OPENJSON(#Json_Array)
WITH (Employee_ID int, Employee_Name nvarchar(60), SSN int)
-- scenario 2: SSN is specified with null for the value
SET #Json_Array = '[{"Employee_ID":123,"Employee_Name":"John Doe","SSN":null}]';
SELECT *
FROM OPENJSON(#Json_Array)
WITH (Employee_ID int, Employee_Name nvarchar(60), SSN int)
-- scenario 3: SSN is not specified
SET #Json_Array = '[{"Employee_ID":123,"Employee_Name":"John Doe"}]';
SELECT *
FROM OPENJSON(#Json_Array)
WITH (Employee_ID int, Employee_Name nvarchar(60), SSN int)
Both scenario 2 and 3 return a NULL value for SSN.
How can I determine if SSN was not included in the #Json_Array? The reason I need to do this is to avoid updating the employee's SSN to NULL if SSN is not included in the JSON.

Answer 1

The answer is to select [value] from OPENJSON(#json_item) where [key]='Column_Name'. Below is a full example.
-- create table of original data that will be modified
DECLARE #Json_Array_Original nvarchar(max) = '[
{"Employee_ID":123,"Employee_Name":"John Doe","SSN":123456789},
{"Employee_ID":124,"Employee_Name":"John Smith","SSN":343434343},
{"Employee_ID":125,"Employee_Name":"Jim Doe","SSN":515151515}
]';
declare #Original_Data table (Employee_ID int, Employee_Name nvarchar(20), SSN int);
insert into #Original_Data
select * from openjson(#Json_Array_Original)
with (Employee_ID int, Employee_Name nvarchar(20), SSN int)
select * from #Original_Data
-- create table of data changes that will be applied
DECLARE #Json_Array_Changes nvarchar(max) = '[
{"Employee_ID":123,"Employee_Name":"Jon Doe","SSN":null},
{"Employee_ID":124,"Employee_Name":"John Smythe"},
{"Employee_ID":125,"SSN":555151515}
]';
declare #Data_Changes table (Employee_ID int, Employee_Name nvarchar(20), SSN int);
insert into #Data_Changes
select * from openjson(#Json_Array_Changes)
with (Employee_ID int, Employee_Name nvarchar(20), SSN int)
select * from #Data_Changes
-- create table containing json records of data changes
declare #Json_Items table ([key] int, [value] nvarchar(max), type int);
insert into #Json_Items
select * from openjson(#Json_Array_Changes)
-- create table that specifies with a 1 the columns (properties) of each json record that have new values provided
declare #Json_Keys table (Employee_ID int, Employee_Name bit, SSN bit);
insert into #Json_Keys
select
json_value([value], '$.Employee_ID'),
(select 1 from openjson([value]) where [key]='Employee_Name'),
(select 1 from openjson([value]) where [key]='SSN')
from #Json_Items
select * from #Json_Keys
-- apply new values only for columns (properties) that have values specified
update od
set
od.Employee_Name = iif(jk.Employee_Name = 1, dc.Employee_Name, od.Employee_Name),
od.SSN = iif(jk.SSN = 1, dc.SSN, od.SSN)
from #Original_Data od
inner join #Data_Changes dc on od.Employee_ID = dc.Employee_ID
inner join #Json_Keys jk on dc.Employee_ID = jk.Employee_ID
select * from #Original_Data