sql join - Wikitechy

What is the Difference between join and union ?

Editor — Sun, 18 Jul 2021 20:27:46 +0000

Union

- The union operator combines the results of two or more queries into a single result set. But no.of columns must match in both/all the queries (and also the order) which are used for union.
- We can union two tables irrespective of common field.
- The UNION set operator is used for combining data from two tables which have columns with the same datatype.
  - Union – Returns with no duplicate rows.
  - Union all – Retruns with duplicate rows (No. of rows returned = No. of rows in Query1 + No. of rows in Query 2).

We cannot use the union operator within a create view statement.
We cannot use the union operator on text and image columns.

Syntax

SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];

Join

Joins are used to extract information from more than one table based on the related column/coloums (PK and RFK) any no. of rows can be retrived based on matching colums.
We can join two tables by ‘join’ if they have common field.
A Join is used for displaying columns with the same or different names from different tables.
Different types of joins are Inner join=equi join=join , outer join (Right outer join/right join, Left outer join / left join), Cross join , and full outer join/outer join.
We can use join operator within a create view statement.
We can use the join operator on text and image columns.

Syntax

SELECT column-names
FROM table-name1 JOIN table-name2 
ON column-name1 = column-name2
WHERE condition

What is the difference between a HAVING CLAUSE and a WHERE CLAUSE ?

Editor — Sun, 18 Jul 2021 20:21:49 +0000

Where Clause	Having Clause
WHERE clause can be used with – Select, Insert, and Update statements.	HAVING clause can only be used with the Select statement.
The WHERE search condition is applied before the grouping operation occurs.	The HAVING search condition is applied after the grouping operation occurs.
Aggregate functions cannot be used in the WHERE clause, unless it is in a sub query contained in a HAVING clause.	Aggregate functions can be used in Having clause.

Example of WHERE and HAVING clause

We have two tables Wikitechy_Employee and Department. Wikitechy_Employee contains details of employees e.g. id, name, age, salary and department id, while Department contains id and department name.
In order to show, which employee works for which department we need to join two tables on DEPT_ID to get the the department name.
Our requirement is to find how many employees are working in each department and average salary of department.
In order to use WHERE clause, we will only include employees who are earning more than 5000.
Before executing our query which contains WHERE, HAVING, and GROUP BY clause, let see data from Wikitechy_Employee and Department table:

SELECT * FROM Wikitechy_Employee;

EMP_ID	EMP_NAME	EMP_AGE	EMP_SALARY	DEPT_ID
1	George Orwell	23	10000	1
2	J.K. Rowling	24	7000	2
3	Kurt Vonnegut	25	8000	3
4	Virginia Woolf	27	6000	1
5	Ernest Hemingway	28	5000	2

SELECT * FROM Department;

DEPT_ID	DEPT_NAME
1	Accounting
2	Marketing
3	Sales

Sample Code

 
SELECT d.DEPT_NAME,
count(e.EMP_NAME) as NUM_EMPLOYEE, 
avg(e.EMP_SALARY) as AVG_SALARY FROM Employee e,
Department d WHERE e.DEPT_ID=d.DEPT_ID AND EMP_SALARY > 5000 GROUP BY d.DEPT_NAME;

Output

DEPT_NAME	NUM_EMPLOYEE AVG_SALARY	AVG_SALARY
Accounting	1	8000
Marketing	1	7000
Sales	2	8000

From the number of employee (NUM_EMPLOYEE) column you can see that only Vijay who work for Marketing department is not included in result set because his earning 5000.
This example shows that, condition in WHERE clause is used to filter rows before you aggregate them and then HAVING clause comes in picture for final filtering, which is clear from following query, now Marketing department is excluded because it doesn’t pass condition in HAVING clause i..e AVG_SALARY > 7000

Sample Code

 
SELECT d.DEPT_NAME,
count(e.EMP_NAME) as NUM_EMPLOYEE,
avg(e.EMP_SALARY) as AVG_SALARY FROM Employee e,
Department d WHERE e.DEPT_ID=d.DEPT_ID AND EMP_SALARY > 5000 GROUP BY d.DEPT_NAMEHAVING AVG_SALARY > 7000;

Output

DEPT_NAME	NUM_EMPLOYEE	AVG_SALARY
Accounting	1	8000
Sales	2	8000

What is an Index ?

Editor — Sun, 18 Jul 2021 20:16:01 +0000

What is an Index ?

Index is a database object which improves the efficiency of the query. When queries are run against a database, an index on that database basically helps in the way the data is sorted to process the query for faster and data retrievals are much faster when we have an index.