Introduction to MySQL Queries
There are many kinds of SQL commands which can be categorized into the following:
- DDL (Data definition language)
- DML (Data manipulation language)
- DQL (Data query language)
- DCL (Data control language)
- TCL (Transaction control language)
In this article of MySQL Queries. We are going to discuss mostly DQL which is “Data Query Language”. This comes to play when we try to fetch records from the database and it starts with the “SELECT” command. Apart from this, we will discuss the brief significance of other categories as well.
Types of MySQL Queries
Following are the 5 types of Queries are:
DDL
When we perform any changes with the physical structure of the table in the database, then we need DDL commands. CREATE, ALTER, RENAME, DROP, TRUNCATE, etc commands come into this category. Those commands can’t be rolled back.
1. CREATE: It used to create a table or database.
Query:
CREATE table employee;
2. ALTER: Used to modify or change values in the table.
Query:
ALTER TABLE table_name
ADD COLUMN col_name;
3. RENAME: Rename the table or database name.
Query:
ALTER TABLE table_name
RENAME COLUMN col_name TO new_col_name;
4. DROP: This removes records of a table as well as the structure of a table. This can’t be rolled back/undo
Query:
DROP TABLE IF EXISTS table_name;
5. TRUNCATE: This empties the records only, leaves the structure for future records.
Query:
TRUNCATE TABLE employee;
DML
As we can see the name Data Manipulation language, so once the tables/database are created, to manipulate something inside that stuff we require DML commands. Merits of using these commands are if incase any wrong changes happened, we can roll back/undo it.
1. INSERT: Used to insert new rows into the table.
Query:
INSERT into employee
Values(101,’abcd’);
2. DELETE: Used to delete a single row or entire records in a table.
Query:
DELETE TABLE employee;
3. UPDATE: Used to update existing records in a table.
Query:
UPDATE employee
SET col1 = new_col
WHERE condition;
4. MERGE: Used to merge two rows.
DCL
It grants or revokes access of users to the database.
1. GRANT: Provides access to the users
Query:
GRANT CREATE table to user;
2. REVOKE: Take access back from the users
Query:
REVOKE CREATE table from user;
TCL
This manages the issues related to the transaction in any database. This is used to rollback or commit in the database.
1. ROLLBACK: Used to cancel or undo changes made in the database
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Query:
ROLLBACK;
2. COMMIT: Used to deploy changes in the database
Query:
COMMIT;
DQL
Data query language consists of only SELECT command by which we can retrieve and fetch data on the basis of some conditions provided. Many clauses of SQL are used with this command for retrieval of filtered data.
1. SELECT: Used to fetch all the records from a table
Query:
SELECT * FROM table;
2. DISTINCT: Used to fetch all the unique values from a table
Query:
SELECT DISTINCT col_name FROM table;
3. WHERE: Used forgiving conditions in the retrieval of records
Query:
SELECT employee_id FROM employee
WHERE name = ‘stella’;
4. COUNT: Used to get the number of records present in a table
Query:
SELECT COUNT(*) FROM employee;
5. ORDER BY: Used to sort both numeric and string values either in ascending or descending manner. But by default, it sorts in an ascending manner. If we want in descending, then we need to specify it after using the ORDER BY clause.
Query:
SELECT first_name FROM student
ORDER BY marks desc;
6. LIMIT: This is used to specify the number of records we want after executing the query. If we want the top 5 students of a class, then after sorting the results, we can use this LIMIT by specifying 5. So that it will only fetch the top 5 records.
Query:
SELECT first_name FROM student
ORDER BY marks desc
LIMIT 5;
(**ORDER BY used here for sorting value in descending order)
7. AND: If 2 conditions are given and both are met for a record, then only the query will fetch that records.
Query:
SELECT employee_id FROM employee
WHERE name = ‘stella’ AND city = ‘Bangalore’;
8. OR: If 2 conditions are given and one of them is met for a record, then that record will be fetched.
Query:
SELECT employee_id FROM employee
WHERE department = ‘IT’ OR city = ‘Bangalore’;
9. NOT: Used with conditions. If we specify NOT before any conditions, records that are not meeting that conditions will be fetched.
Query:
SELECT employee_id FROM employee
WHERE NOT BETWEEN 1 AND 10;
10. BETWEEN: This operator selects records within a given range. Mostly we use this where we want to specify a range of dates.
Query:
SELECT emp_id FROM employee
WHERE emp_id BETWEEN 1 AND 10;
Query:
SELECT * FROM employee
WHERE join_date BETWEEN '2007-01-01' AND '2008-01-01';
11. IN: This operator allows us to specify multiple values in a WHERE clause.
Query:
SELECT * FROM employee
WHERE employee_id IN (1001,1004,1008,1012);
12. LIKE: This operator is used with the WHERE clause to search for a specified pattern in a column containing a string. ‘A%’ – string starts with A
- ‘&A’: ends with A
- ‘%A%’: A will be in between the string
- ‘_A%’: Here the 2nd letter will be A
- ‘%A_’: The 2nd from the last letter will be A
Query:
SELECT first_name FROM table
WHERE first_name LIKE 'A%';
13. SUBSTRING: Used to pick a specific character from a string by specifying a position
Query:
SELECT SUBSTRING(customer_name,1,5) FROM customer_table;
(it will fetch character from 1st to 5th position of a string)
14. INSTR: This returns a position of a string in another string
Query:
SELECT INSTR(‘independence’, ‘pen’);
(it will find the position of ‘pen’ in the word ‘independence’)
15. GROUP BY: This is used to segregate records on the basis of some given conditions
Query:
SELECT employee_id FROM employee GROUP BY department HAVING salary > 100000;
(Here group by segregated employees on the basis of their department and whose salary more than 100k.
Condition always comes with HAVING statement in GROUP BY clause.)
Aggregated Function
16. SUM: Calculates sum of values
Query:
SELECT SUM(salary) FROM employee;
17. AVG: Calculates the average of set of value
Query:
SELECT AVG(salary) FROM employee;
18. MIN: Gets the minimum value in a set of values
Query:
SELECT MIN(salary) FROM employee;
19. MAX: Gets the maximum value in a set of values
Query:
SELECT MAX(salary) FROM employee;
Joins
20. INNER JOIN: Returns records that have a matching value in both tables
Query:
SELECT * FROM order
INNER JOIN customer
ON order.cust_id = customer.cust_id;
21. LEFT JOIN: Returns all records from left table and the matched records from the right table
Query:
SELECT * FROM order
LEFT JOIN customer
ON order.cust_id = customer.cust_id;
22. RIGHT JOIN: Returns all records from right table and the matched records from the left table
Query:
SELECT * FROM order
RIGHT JOIN customer
ON order.cust_id = customer.cust_id;
23. FULL OUTER JOIN: Returns all the records when there is a match in either left or right table
Query:
SELECT * FROM order
FULL OUTER JOIN customer
ON order.cust_id = customer.cust_id;
Conclusion
These commands and clauses we have discussed above are very useful in real-time scenarios as it provides the basic concepts of how to use SQL queries to fetch and manipulate data in the database. Apart from this, while using advance and analytical queries like window function etc, these clauses are very important.
Recommended Articles
This is a guide to MySQL Queries. Here we discuss the basic concept, types of Queries in MySQL which include DDL, DML, DQL, and DCL, TCL. You can also go through our other suggested articles to learn more –
