MySQL JOINS allow retrieving data from two or more related database tables using primary and foreign keys. There are several types of JOINs: Cross JOINs return all combinations of rows from both tables; Inner JOINs return rows where the joined columns are equal; Left and Right JOINs return all rows from the left or right table respectively and matching rows from the other table. JOINs improve performance over multiple queries by using indexing and reducing data transfers between MySQL and applications.
2. What are JOINS?
Joins help retrieving data from two or more database tables.
The tables are mutually related using primary and foreign keys.
3. MySQL Cross Join
Cross JOIN is a simplestform of JOINs which matches each row from one databasetable to all
rows of another.
In other words it gives us combinations of each row of first table with all records in second table.
Cross Join
All rows from both tables
4. MySQL Cross Join
All mentioned bellow lines will produce the same result:
SELECT * FROM table_1 CROSS JOIN table_2;
OR
SELECT table_1.id, table_1.greeting, table_2.id, table_2.question FROM table_1 CROSS JOIN table_2;
OR
SELECT table_1.id, table_1.greeting, table_2.id, table_2.question FROM table_1 , table_2;
5. MySQL Cross Join
id greeting
1 hello
2 hi
id question
1 How are you doing?
2 Are you there?
table_1 table_2
SELECT * FROM table_1 CROSS JOIN table_2;
Result
1
2
1
2
6. MySQL Inner Join
The inner JOIN is used to return rows from both tables that satisfy the given condition.
The inner JOIN is same as JOIN clause, combining rows from two or more tables.
Inner Join
Only matching rows
7. MySQL Inner Join
Both lines will produce the same result:
SELECT * FROM table_1 INNER JOIN table_2 ON table_1.id = table_2.id;
OR
SELECT * FROM table_1 JOIN table_2 ON table_1.id = table_2.id;;
8. MySQL Inner Join
id greeting
1 hello
2 hi
id question
1 How are you doing?
2 Are you there?
table_1 table_2
SELECT * FROM table_1 INNER JOIN table_2 ON table_1.id = table_2.id;
Result
10. MySQL Left Join
The LEFT JOIN returns all the rows from the table on the left even if no matching rows
have been found in the table on the right.
Where no matches have been found in the table on the right, NULL is returned
Left Join
All rows from the left table
11. MySQL Left Join
Both lines will produce the same result:
SELECT * FROM table_1 LEFT JOIN table_2 ON table_1.id = table_2.id;
OR
SELECT * FROM table_1 LEFT OUTER JOIN table_2 ON table_1.id = table_2.id;;
12. MySQL Left Join
id greeting
1 hello
2 hi
3 hey
id question
1 How are you doing?
2 Are you there?
table_1 table_2
SELECT * FROM table_1 LEFT JOIN table_2 ON table_1.id = table_2.id;
Result
13. MySQL Right Join
The RIGHT JOIN returns all the columns from the table on the right even if no matching
rows have been found in the table on the left.
Where no matches have been found in the table on the left, NULL is returned.
Right Join
All rows from the right table
14. MySQL Right Join
Both lines will produce the same result:
SELECT * FROM table_1 RIGHT JOIN table_2 ON table_1.id = table_2.id;
OR
SELECT * FROM table_1 RIGHT OUTER JOIN table_2 ON table_1.id = table_2.id;;
15. MySQL Right Join
id greeting
1 hello
2 hi
id question
1 How are you doing?
2 Are you there?
3 Are you ready?
table_1 table_2
SELECT * FROM table_1 RIGHT JOIN table_2 ON table_1.id = table_2.id;
Result
16. MySQL Right Join with USING clause
id question
1 How are you doing?
2 Are you there?
3 Are you ready?
table_1 table_2
SELECT * FROM table_1 RIGHT JOIN table_2 USING(id);
Result
id greeting
1 hello
2 hi
17. Why we use JOINs
Using JOINs, you can get the work done by using only a one query with any search
parameters.
MySQL can achieve better performance with JOINs as it can use Indexing. Simply use of single
JOIN query instead running multiple queries do reduce server overhead. Using multiple
queries instead that leads more data transfers between MySQL and applications (software).
Further it requires more data manipulations in application end also.
