How to join two total tables using sql? - sql

Question

For a university work we have two tables in sql:
table1:
column_name1 number_P1
PARIS 10
LISBOA 20
RIO 30
table2:
column_name2 number_P2
PARIS 100
NEW YORK 300
I need to join the two tables by adding the total number of people in each city. So I tried to do:
SELECT table1.column_name1,
number_P2 + number_P1 AS TOTAL
FROM table1
LEFT JOIN table2 ON table1.column_name = table2.column_name;
However, if a city A appears in table 1 and does not appear in table 2 this would not work. The same would happen if a City B appears in table 2 and does not appear in table 1. How can I generalize these situations?
Desired output:
column_name number_P
PARIS 110
LISBOA 20
RIO 30
NEW YORK 300

Answer 1

We can try to use UNION ALL with SUM instead of JOIN
SELECT column_name,
SUM(number_P) number_P
FROM (
SELECT column_name1 as column_name,number_P1 as number_P
FROM table1
UNION ALL
SELECT column_name2,number_P2
FROM table2
) t1
GROUP BY column_name

Answer 2

Another way to achieve this without a subquery.
SELECT IFNULL(table1.column_name1,table2.column_name2) AS ColumnName,
(IFNULL(number_P2,0)+ IFNULL(number_P1,0)) AS TOTAL
FROM table1
FULL JOIN table2 ON table1.column_name1 = table2.column_name2;
Output
ColumnName
TOTAL
PARIS
110
LISBOA
20
RIO
30
NEW YORK
300
To replace 'RIO' with 'RIO DE JANEIRO'
SELECT CASE IFNULL(table1.column_name1,table2.column_name2)
WHEN 'RIO' THEN 'RIO DE JANEIRO'
ELSE IFNULL(table1.column_name1,table2.column_name2) END AS ColumnName,
(IFNULL(number_P2,0)+ IFNULL(number_P1,0)) AS TOTAL
FROM table1
FULL JOIN table2 ON table1.column_name1 = table2.column_name2;