Question

i have a table called users contained

(Table A) 
Users
user_id
username
user_type[1,2]

(Table B)     if user_type=1       
user_id
full_name

(Table C)     if user_type=2        
user_id 
full_name

i want to get single record set by executing single query, is that possible in PHP mysql.

Answer 1

Try this:

SELECT table_a.*, COALESCE(table_b.full_name,table_c.full_name) AS full_name
FROM table_a
LEFT OUTER JOIN table_b ON table_b.user_id = table_a.user_id
LEFT OUTER JOIN table_c ON table_c.user_id = table_a.user_id WHERE 1;

It uses the LEFT OUTER JOIN, which means that it joins it to table_b on the given condition. However, for each row in table_a, whether it finds a matching row in table_b or not, it will return the table_a row. If it does not find a matching row, the table_b columns are just NULL. Same thing with table_c.

Then, we just select all the table_a columns. However, we now have two full_name columns, one from table_b and one from table_c. We use COALESCE to merge them.

COALESCE returns the first non-NULL value.

Since we know that there is either a matching row in table_b or a matching row in table_c, it is not a problem. However, it would be a problem if somehow you allows a matching row to be found in both table_b and table_c.

The risk can be mitigated by adding additional ON clause conditions to get:

SELECT table_a.*, COALESCE(table_b.full_name,table_c.full_name) AS full_name
FROM table_a
LEFT OUTER JOIN table_b
             ON table_b.user_id = table_a.user_id AND table_a.user_type = 1
LEFT OUTER JOIN table_c
             ON table_c.user_id = table_a.user_id AND table_a.user_type = 2
WHERE 1;

Still, you will need to make sure only 1 row is present for each user in table_b and table_c.

---

Instead of COALESCE you can optionally use CASE like:

SELECT table_a.*, CASE user_type WHEN 1
        THEN table_b.full_name
        ELSE table_c.full_name END AS full_name
...

or use an IF function like:
SELECT table_a.*, IF(user_type=1,table_b.full_name,table_c.full_name) AS full_name
…