This document provides an overview of JOINs and UNIONS in MySQL. It defines different types of JOINs (INNER JOIN, LEFT JOIN, RIGHT JOIN) and provides syntax examples to retrieve data from multiple tables based on relationships between columns. While FULL JOIN is not supported in MySQL, the document demonstrates how to emulate it using a UNION of a LEFT JOIN and RIGHT JOIN. It also covers the UNION and UNION ALL operators to combine result sets from multiple queries.

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3. JOINS & UNIONS IN
MYSQL
ARUN KUMAR K A
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4. JOIN
• To query data from two or more tables, based on a relationship between
certain column in these tables.
• Tables in a database are often related to each other with keys.
• Primary key is a column (or a combination of columns) with a unique
value for each row.
• Foreign Key in one table points to a Primary Key in another table.

5. Different JOINs in MySQL
INNER JOIN(JOIN):
Return rows when there is at least one match in both tables.
LEFT JOIN:
Return all rows from the left table, even if there are no matches
in the right table.
RIGHT JOIN:
Return all rows from the right table, even if there are no matches in the left
table.
FULL JOIN:
Return rows when there is a match in one of the tables.
But It does not support in MySQL.

6. Tbl_dept
pk_dept_id dept_name
1 CSE
2 ECE
3 EEE
4 CE

7. tbl_student
pk_student_id student_name student_place fk_dept_id
1 baabtra1 Calicut 1
2 baabtra2 cochin 2
3 baabtra3 Kannur 3
4 baabtra4 ooty 3
5 baabtra5 munnar 1
6 baabtra6 mlp NULL

8. INNER JOIN (JOIN)
• The INNER JOIN keyword returns rows when there is at least one match in
both tables.
Syntax:
SELECT
column name(s)
FROM
table_name1
INNER JOIN
table_name2
ON
table_name1.column_name=table_name2.column_name
Tbl_dept Tbl_student

9. INNER JOIN Example
student_name dept_name
baabtra1 CSE
baabtra5 CSE
baabtra2 ECE
baabtra3 EEE
baabtra4 EEE
SELECT tbl_student.student_name,tbl_dept.dept_name
FROM tbl_dept
JOIN tbl_student
ON tbl_dept.pk_dept_id = tbl_student.fk_dept_id;

10. LEFT JOIN
• The LEFT JOIN keyword returns all rows from the left table (table_name1),
even if there are no matches in the right table (table_name2).
Syntax:
SELECT
column_name(s)
FROM
table_name1
LEFT JOIN
table_name2
ON
table_name1.column_name=table_name2.column_name.

11. LEFT JOIN Example
SELECT tbl_student.student_name,tbl_dept.dept_name
FROM tbl_student
LEFT JOIN tbl_dept
ON tbl_dept.pk_dept_id = tbl_student.fk_dept_id;
Student_name Dept_name
Baabtra1 CSE
Baabtra2 ECE
Baabtra3 EEE
Baabtra4 EEE
Baabtra5 CSE
Baabtra6 NULL

12. RIGHT JOIN
• The RIGHT JOIN keyword returns all the rows from the right table
(table_name2), even if there are no matches in the left table (table_name1).
Syntax:
SELECT
column_name(s)
FROM
table_name1
RIGHT JOIN
table_name2
ON
table_name1.column_name=table_name2.column_name.

13. RIGHT JOIN Example
SELECT tbl_student.student_name,tbl_dept.dept_name
FROM tbl_student
RIGHT JOIN tbl_dept
ON tbl_dept.pk_dept_id = tbl_student.fk_dept_id;
student_name dept_name
Baabtra1 CSE
Baabtra5 CSE
Baabtra2 ECE
Baabtra3 EEE
Baabtra4 EEE
NULL CE

14. FULL JOIN
• There is no FULL JOIN in MySQL.
• We can possible this by using both LEFT and RIGHT Joins together with
UNION key word.
Tbl_dep Tbl_student

15. UNION
• The UNION operator combines two or more SELECT statements.
• The UNION operator is used to combine the result-set of two or more
SELECT statements.
• Each SELECT statement within the UNION must have the same number of
columns.
• The columns must also have similar data types.
• The columns in each SELECT statement must be in the same order.

16. UNION Syntax
SELECT column _ name(s)
FROM Table_name1
UNION
SELECT column _ name(s)
FROM table_name2
• The column names in the result-set of a UNION are always equal to the
column names in the first SELECT statement in the UNION.

17. SELECT tbl_dept.dept_name
FROM tbl_dept
UNION
SELECT tbl_student.student_name
FROM tbl_student;
dept_name
CSE
ECE
EEE
CE
Baabtra1
Baabtra2
Baabtra3
Baabtra4
Baabtra5
baabtra6

18. UNION ALL
• The UNION operator selects only distinct values by default. If you need to
allow duplicate values, use UNION ALL.
Syntax
SELECT column_name (s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2

19. UNION Example
SELECT tbl_student.student_name,tbl_dept.dept_name
FROM tbl_dept
LEFT JOIN tbl_student
ON tbl_dept.pk_dept_id = tbl_student.fk_dept_id
UNION
SELECT tbl_student.student_name,tbl_dept.dept_name
FROM tbl_dept
RIGHT JOIN tbl_student
ON tbl_dept.pk_dept_id = tbl_student.fk_dept_id;

20. Student_name Dept_name
Baabtra1 CSE
Baabtra5 CSE
Baabtra2 ECE
Baabtra3 EEE
Baabtra4 EEE
NULL CE
Baabtra6 NULL

21. UNION ALL Example
SELECT tbl_student.student_name,tbl_dept.dept_name
FROM tbl_dept
LEFT JOIN tbl_student
ON tbl_dept.pk_dept_id=tbl_student.fk_dept_id
UNION ALL
SELECT tbl_student.student_name,tbl_dept.dept_name
FROM tbl_dept
RIGHT JOIN tbl_student
ON tbl_dept.pk_dept_id=tbl_student.fk_dept_id;

22. Student_name Dept_name
Baabtra1 CSE
baabtra5 CSE
baabtra2 ECE
baabtra3 EEE
baabtra4 EEE
NULL CE
baabtra1 CSE
baabtra2 ECE
baabtra3 EEE
baabtra4 EEE
baabtra5 CSE
baabtra6 NULL

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