Replace values based on JOIN

Let’s say I have two data sets A and B

After left join I should get FALSE value for Y in table A as it doesn’t have it in table B

Answer

Using your approach, if you intend to get a value of FALSE if the value doesn’t exist you can check for NULL in the name. The example below uses a CASE to check for NULL and assigns the original value otherwise

SELECT 
  A.Name,
  CASE 
     WHEN B.Name is NULL THEN 'FALSE'
     ELSE A.Value
  END as Value
FROM
  A
LEFT JOIN
  B on A.Name = B.Name