Merge tables by inserting null values for columns where no value can be derived from the merging tables?

Question

I have two different tables and i decided to merge them into one table...

the birth table has

id, name, country, birthday_date, description, link

the death table has

id, name, country, death_date, description, link

and i want to merge them into a single table with the structure

id, name, country, bdate, ddate, description, link.

The link from each table has a unique value so i have to merge the tables using the link. I tried many queries but resulted in wrong results.

Both the birth and death table can have same names and some names may have present in either only birth or death table.

If then, How can i merge them and updating a null date for a column that has no value on any of the two old tables?

Answer 1

Assuming that your new id has AUTO_INCREMENT this solution using CROSS JOIN and COALESCE should work for you. name, country and description from death will be ignored, if available in birth.

INSERT INTO new_table ( name, country, bdate, ddate, description, link )
(
  SELECT
    COALESCE(b.name, d.name),
    COALESCE(b.country, d.country),
    b.birthday_date,
    d.death_date,
    COALESCE(b.description, d.description),
    COALESCE(b.link, d.link)
  FROM birth b
  CROSS JOIN death d ON ( d.link = b.link )
)

---

Alternatively, insert all rows from birth:

INSERT INTO new_table ( name, country, bdate, ddate, description, link )
(
  SELECT name, country, birthday_date, NULL, description, link
  FROM birth
)

Then update those rows with a `death_date´ if available:

UPDATE new_table nt
SET ddate = (SELECT death_date FROM death d WHERE d.link = nt.link )

Finally, add all rows from death, that have no entry in birth:

INSERT INTO new_table ( name, country, bdate, ddate, description, link )
(
  SELECT name, country, NULL, death_date, description, link
  FROM death d
  WHERE NOT EXISTS ( SELECT NULL
                     FROM new_table
                     WHERE link = d.link
                   )
)