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SQL Interview Questions and Answers

Be First!
by October 1, 2015 MySQL

sql1.What is the difference between JOIN and UNION?
SQL JOIN allows us to “lookup” records on other table based on the given conditions between two tables. For example, if we have the department ID of each employee, then we can use this department ID of the employee table to join with the department ID of department table to lookup department names.

UNION operation allows us to add 2 similar data sets to create resulting data set that contains all the data from the source data sets. Union does not require any condition for joining. For example, if you have 2 employee tables with same structure, you can UNION them to create one result set that will contain all the employees from both of the tables.

SELECT * FROM EMP1
UNION
SELECT * FROM EMP2;

2.What is the difference between UNION and UNION ALL?
UNION and UNION ALL both unify for add two structurally similar data sets, but UNION operation returns only the unique records from the resulting data set whereas UNION ALL will return all the rows, even if one or more rows are duplicated to each other.

In the following example, I am choosing exactly the same employee from the emp table and performing UNION and UNION ALL. Check the difference in the result.

SELECT * FROM EMPLOYEE WHERE ID = 5
UNION ALL
SELECT * FROM EMPLOYEE WHERE ID = 5

3.What is the difference between WHERE clause and HAVING clause?
WHERE and HAVING both filters out records based on one or more conditions. The difference is, WHERE clause can only be applied on a static non-aggregated column whereas we will need to use HAVING for aggregated columns.

To understand this, consider this example.
Suppose we want to see only those departments where department ID is greater than 3. There is no aggregation operation and the condition needs to be applied on a static field. We will use WHERE clause here:

SELECT * FROM DEPT WHERE ID > 3

SELECT dept.name DEPARTMENT, avg(emp.sal) AVG_SAL
FROM DEPT dept, EMPLOYEE emp
WHERE dept.id = emp.dept_id (+)
GROUP BY dept.name
HAVING AVG(emp.sal) > 80

4.How to select first 5 records from a table?
This question, often asked in many interviews, does not make any sense to me. The problem here is how do you define which record is first and which is second. Which record is retrieved first from the database is not deterministic. It depends on many uncontrollable factors such as how database works at that moment of execution etc. So the question should really be – “how to select any 5 records from the table?” But whatever it is, here is the solution:

In Oracle,

SELECT *
FROM EMP
WHERE ROWNUM <= 5;
In SQL Server,

SELECT TOP 5 * FROM EMP;

5.How to generate row number in SQL Without ROWNUM?
Generating a row number – that is a running sequence of numbers for each row is not easy using plain SQL. In fact, the method I am going to show below is not very generic either. This method only works if there is at least one unique column in the table. This method will also work if there is no single unique column, but collection of columns that is unique. Anyway, here is the query:

SELECT name, sal, (SELECT COUNT(*) FROM EMPLOYEE i WHERE o.name >= i.name) row_num
FROM EMPLOYEE o
order by row_num

 

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