Showing posts with label SQL Exercise. Show all posts
Showing posts with label SQL Exercise. Show all posts

Thursday, September 1, 2022

SQL Cheatsheet - SELECT FROM WHERE clauses with examples

SQL Cheat sheet for beginners, SELECT clause, FROM clause, WHERE clause explained with examples.

 

 SELECT ... FROM ... WHERE clauses in SQL - Cheat sheet


Querying data using SELECT…FROM...WHERE clauses in SQL

Commands/Clauses

Examples


SELECT 👈

To view required data from one or more tables.

 

Syntax:

SELECT parameters FROM tablename;

 

Parameters for SELECT:

*, list of attributes, functions, arithmetic operations are the parameters for the command (clause) SELECT.

 

 

 

 

 

 

 

 


Parameters examples for SELECT clause

Simple:

SELECT * FROM student;

Result: Displays all columns of all records of student table.


List of attributes:

SELECT name, phone FROM student;

Result: Displays only name and phone of all students;


Function:

SELECT MAX(salary) FROM employee;

Result: Displays the maximum of all salary values stored in employee table.


Operations:

SELECT Basic + HRA FROM employee;

Result: Displays the sum of the values stored in Basic and HRA columns (attributes) of all employee records.

 


FROM 👈

To specify the name of the table/view from which we want to view the data.

 

Syntax:

SELECT parameters FROM tablename;


Parameters for FROM:

Table name or table names separated with comma.

 

 


Parameters examples for FROM clause

Single table:

SELECT * FROM employee;

Result: Displays all records from employee table.


Multiple tables:

SELECT * FROM employee, dept;

Result: It is a special case. It performs join. Displays columns of tables, employee and dept with every record of table employee combined with every record of dept.


WHERE 👈

To specify one or more conditions to filter the data to view/get required data only.

 

Syntax:

SELECT parameters FROM tablename WHERE condition;

 

Parameters for WHERE:

WHERE clause accepts the conditions (predicates) of the following format;

Attribute_name OP value;

Attribute_name OP attribute_name;

Here, OP refers to OPERATOR that can be one of =, <>, >, <, >=, and <=

 

More conditions can be mentioned in WHERE clause. In that case, the conditions should be connected using logical connectives AND, OR, NOT.


Parameter examples for WHERE clause

attribute_name OP value:

SELECT * FROM employee WHERE gender = ‘male’;

Result: Displays all records of employee who are MALE.

 

SELECT * FROM employee WHERE salary > 10000;

Result: Displays all records of employees who earn more than 10000.

 

SELECT name, age FROM employee WHERE salary = 25000;

Result: Displays only name and ages of all employees who earn 25000.


attribute_name OP attribute_name:

SELECT * FROM employee, dept WHERE emp_dno = dept_dno;

 

 

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What are the parameters for SELECT clause?

Can we perform arithmetic calculations using SELECT clause in SQL?

How to filter the data in a table using WHERE clause?

Can we use more than one condition in WHERE clause?

How do you find cars that are sold for fifty thousand dollars or more?

Where to specify the condition like the watches that are costlier than Omega watches? 

Wednesday, January 26, 2022

DBMS solved MCQ - Find equivalent queries to the given SQL query

Multiple choices questions in DBMS, Structured Query Language, Find equivalent alternate queries for the given SQL queries, Can we write equivalent alternate queries to an SQL query?

DBMS MCQ - Suggesting alternate equivalent queries to an SQL query

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1. Consider a table STU with schema STU(Regno, Name, DOB, Phone) and Regno as the primary key. Which of the following queries is equivalent to the query "SELECT * FROM stu WHERE Regno=1 AND Phone=2;" Choose all queries that are equivalent.

a) select * from stu where regno=1 and phone=(select phone from stu where phone=2 order by phone fetch first 1 rows only);

b) select * from stu where regno=1 and phone in (select phone from stu where phone=2);

c) select * from stu where regno=1 and phone in (1, 2);

d) select * from stu where phone=2 and regno in (1, 2, 3);

Answer: (a) select * from stu where regno=1 and phone=(select phone from stu where phone=1 order by phone fetch first 1 rows only); and (b) select * from stu where regno=1 and phone in (select phone from stu where phone=2);

The query given in the question finds and displays all records that have 1 in register number attribute (column) and 2 in phone attribute from STU table.

Option (a) does the same thing using a sub-query. The sub-query here fetches all the phone numbers that are equivalent to 2. As phone attribute is not a key attribute, the result may consist of more than 1 entry. Hence, we use the ORDER BY clause and FETCH to include only the top 1 result.

Option (b) is does the same thing as the given query hence it is EQUIVALENT.

Option (c) is NOT EQUIVALENT because it accepts the record (1, 1) also which is not expected as per the given query.

Option (d) is NOT EQUIVALENT because it accepts records (1, 2), (2, 2) and (3, 2).

 



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Related posts:


Why is it important to find alternate queries to an SQL query? - because it might help in finding the most optimal query to request the data

How to write alternate queries to the SQL query? 

Find alternate equivalent queries to the given SQL query

Structured query language query and its equivalent

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