The principal deviation betwixt Self Join together with Equi Join is that In Self Join nosotros bring together ane tabular array to itself rather than joining 2 tables. Both Self Join together with Equi Join are types of INNER Join inwards SQL, but at that spot is the subtle deviation betwixt two. Any INNER Join amongst equal every bit bring together predicate is known every bit Equi Join. SQL Joins are the cardinal concept of SQL similar to correlated together with noncorrelated subqueries or using grouping yesteryear clause together with a proficient agreement of diverse types of SQL bring together is the must for whatever programmer. By the way, If yous convey written INNER bring together using where clause than using comparing operator every bit = volition hold upwards known every bit an equijoin. Equi bring together or Self bring together are non a formal JOIN or component division of the syntax, instead, they are a but pop means to refer sure enough bring together examples.
One of the best instance of Self Join, I convey seen inwards an SQL query Interview questions is "How produce yous notice all Employees who are Managers inwards Employee table", which is usually asked along amongst or together with then other pop inquiry how to notice minute highest salary of employee or questions related to join iii tables inwards ane SQL query.
In this SQL tutorial, nosotros volition larn self-join yesteryear instance piece solving this SQL query.
One of the best instance of Self Join, I convey seen inwards an SQL query Interview questions is "How produce yous notice all Employees who are Managers inwards Employee table", which is usually asked along amongst or together with then other pop inquiry how to notice minute highest salary of employee or questions related to join iii tables inwards ane SQL query.
In this SQL tutorial, nosotros volition larn self-join yesteryear instance piece solving this SQL query.
Self Join Example inwards SQL
In gild to solve this query let's starting fourth dimension run across schema together with information of Employee table.
mysql> select * from employee;
+--------+----------+---------+--------+--------+
| emp_id | emp_name | dept_id | salary | mgr_id |
+--------+----------+---------+--------+--------+
| 103 | Jack | 2 | 1400 | 104 |
| 104 | John | 2 | 1450 | 104 |
| 105 | Johnny | 3 | 1050 | 104 |
| 108 | Alan | 3 | 1150 | 104 |
| 106 | Virat | 4 | 850 | 105 |
| 107 | Vina | 4 | 700 | 105 |
| 109 | joya | 4 | 700 | 105 |
+--------+----------+---------+--------+--------+
7 rows in set (0.00 sec)
+--------+----------+---------+--------+--------+
| emp_id | emp_name | dept_id | salary | mgr_id |
+--------+----------+---------+--------+--------+
| 103 | Jack | 2 | 1400 | 104 |
| 104 | John | 2 | 1450 | 104 |
| 105 | Johnny | 3 | 1050 | 104 |
| 108 | Alan | 3 | 1150 | 104 |
| 106 | Virat | 4 | 850 | 105 |
| 107 | Vina | 4 | 700 | 105 |
| 109 | joya | 4 | 700 | 105 |
+--------+----------+---------+--------+--------+
7 rows in set (0.00 sec)
In above table all employees who are managers has at that spot emp_id every bit mgr_id inwards other employees together with yesteryear using SELF JOIN i.e. bring together 2 instances of employee tabular array together with comparing, nosotros tin notice all employees who are managers. Here is the SELECT query example using self-join:
mysql> select distinct e.emp_id, e.emp_name from employee e join employee one thousand on e.emp_id=m.mgr_id;
+--------+----------+
| emp_id | emp_name |
+--------+----------+
| 104 | John |
| 105 | Johnny |
+--------+----------+
2 rows in set (0.00 sec)
+--------+----------+
| emp_id | emp_name |
+--------+----------+
| 104 | John |
| 105 | Johnny |
+--------+----------+
2 rows in set (0.00 sec)
In this instance of self join, nosotros convey joined employee tabular array to itself yesteryear using 2 tabular array aliases e together with m. We convey likewise used distinct keyword to take away duplicates here. You tin likewise tell this is an instance of EQUI JOIN because inwards bring together predicate nosotros convey used = or equal condition. In fact, this ane is an instance of INNER Join, SELF Join, together with EQUI Join at the same time.If yous are confused near syntax of all possible bring together inwards SQL, hither is a handy diagram:
Self Join vs Equi Join
In the curt major deviation betwixt Self Join together with Equi Join inwards SQL is that Self Join requires solely ane tabular array piece most of Equi bring together is status used inwards bring together predicate. Since Equi Join is based on the status for comparison, it tin hap inwards whatever INNER, OUTER or SELF bring together inwards SQL.
That’s all on the difference betwixt Self Join together with Equi Join inwards SQL. Self Join is ane of the of import technique to solve many SQL query related employment where 2 columns of the tabular array comprise the same type of information e.g. hither emp_id together with dept_id are essentially same data.
Further Learning
Difference betwixt primary key together with candidate key inwards tabular array or SQL
P.S. - If yous are looking for online training/course to larn SQL from scratch, I propose yous joining Introduction to SQL yesteryear Jon Flanders. It's ane of the best sourse to larn SQL fundamentals e.g. join, subquery, aggregate functions, window functions, gropuing data, advanced filtering together with SQL query optimization.
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