Question

I used JPA and mysql.
Each table have entity class.I have four tables.

Table1 : student_table

studentId(PK), studentName 
1              jack
2              robert
3              tom
4              smith   

Table2: roll_table

rollId(PK), studentId(FK) 
10001          1
10002          2
10003          3
10004          4

Table3: address_table

addressId(PK)  City          studentId(FK) 
1               Washngton       1
2               NewYork1        2
3               Newyork2        3
4               Wasington2      4

Table4: contact_table

------------------------------------------------
contactId(pk) phoneNumber email studentId(FK) 
------------------------------------------------

----------------------------------------------

Base table is ‘student_table’. ‘studentId’ is primary key for this table.

The remaining 3 table have used this studentId as foreign key.
Totally 3 table contains data. one table doesn’t have any data.

I need to write query for “studentId = 2 used table names and tables count if data exist other tables.
otherwise is there Any other logic to get this information.

Like now the studentId = 2 used two tables. So the result is *(roll_table,address_table)*
Suppose contact table have data with studentId=2,
then the result is *(roll_table,address_table,contact_table)*

Help me.
Thanks in advance

Answer 1

try left join which help you to display all the records from lefttable(studentName), even there are no matches on right table(roll_table,address_table,contact_table)

    SELECT student_table.studentId,student_table.studentName
 FROM student_table
 LEFT JOIN roll_table ON student_table.studentId = roll_table.studentId
 LEFT JOIN address_table ON student_table.studentId = address_table.studentId
 LEFT JOIN contact_table ON student_table.studentId = contact_table.studentId