How to define target table in MySQL query? - mysql

Question

how to fetch user name with user ID?
Suppose I have 3 tables like this and user.type holds 'e' for employer and 'w' for worker.
'e' says that user name is on employer.name and so 'w' says that user name is in worker.name.
employer.id and worker.id are foreign key of user.id.
Is there any way to get user name while having user id (we don't know type yet) with a single query?
Although if there's a problem with this kind of modeling let me know.

Answer 1

To answer your question directly,
SELECT a.*,
IF(a.type = 'e', b.Name, c.Name) PersonName
FROM User a
LEFT JOIN Employer b
ON a.ID = b.ID
LEFT JOIN Worker c
ON a.ID = c.ID
Actually, you can redesign the schema and needing only two tables, the Person and Type
Type
ID (PK)
TypeName
Person
ID (PK)
Name
UserTypeID (FK)
and you can easily get the type for every person,
SELECT a.*,
b.TypeName
FROM Person a
INNER JOIN Type b
ON a.UserTypeID = b.ID
To further gain more knowledge about joins, kindly visit the link below:
Visual Representation of SQL Joins
SAMPLE RECORDS
TYPE
╔════╦══════════╗
║ ID ║ TYPENAME ║
╠════╬══════════╣
║ 1 ║ Employee ║
║ 2 ║ Worker ║
╚════╩══════════╝
PERSON
╔════╦═════════╦════════════╗
║ ID ║ NAME ║ USERTYPEID ║
╠════╬═════════╬════════════╣
║ 1 ║ Stacky ║ 2 ║
║ 2 ║ Horton ║ 1 ║
║ 3 ║ Alam Na ║ 1 ║
║ 4 ║ Aw aw ║ 2 ║
╚════╩═════════╩════════════╝
OUTPUT AFTER THE QUERY HAS BEEN EXECUTED
╔════╦═════════╦════════════╦══════════╗
║ ID ║ NAME ║ USERTYPEID ║ TYPENAME ║
╠════╬═════════╬════════════╬══════════╣
║ 1 ║ Stacky ║ 2 ║ Worker ║
║ 2 ║ Horton ║ 1 ║ Employee ║
║ 3 ║ Alam Na ║ 1 ║ Employee ║
║ 4 ║ Aw aw ║ 2 ║ Worker ║
╚════╩═════════╩════════════╩══════════╝