what is mapping in sql? - sql

Question

how to map tables in sql?
i have three tables . i want to map this tables using sql queries.
student 'table1'
Studnt_id password
123 5525
124 2233
student_info 'table 2'
Studnt_id student_name subject marks
123 gautam maths 90
124 gaurav maths 85
student_personal_info 'table 3'
firstname lastname address
gautam jethva banglore, karnataka
gaurav rathod jamnagar , gujarat
i want to map this three table and show in the result id, student_name , lastname,subject, marks, address

Answer 1

You need to JOIN (map) tables together; Jeff Atwood has a great overview of how these work in his article, "A Visual Explanation of SQL Joins"
In your example above, you should be able to use the studn_id column to link the tables student ('table1') and student_info ('table2') together. The third may be trickier as it doesn't appear to have a guaranteed key back to either of the first two tables.
Example:
SELECT S.student_name, S.subject, S.marks
FROM student S
INNER JOIN student_info SI ON S.Studnt_id = SI.Studnt_id
Will give you all the rows in student that have a matching row in student_info
However, linking the third table, student_personal_info is much harder. Your issue here is that there doesn't seem to be a unique way of pulling in the rows in this table for a specific student - you could try joining on name, but this isn't guaranteed to be unique.
The table student_personal_info really should have the column Studnt_id in it to make unique selection of rows for a specific student possible. Given your specific example, however, the following will work:
SELECT S.student_name, S.subject, S.marks, SPI.lastname, SPI.address
FROM student S
INNER JOIN student_info SI ON S.Studnt_id = SI.Studnt_id
INNER JOIN student_personal_info SPI ON SI.student_name = SPI.firstname
However, if more than one student shares the same first name, you'll get multiple matches.