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We gathered the most popular MySQL interview questions from the easiest to the trickiest, answered them, and provided a short explanation on why things work the way they do. This will hopefully help you make sense of MySQL, its core concepts, operators, and capabilities, which are actually very logical and straight-forward.
1. How Do You Use the MySQL Foreign Key Constraint?
The MySQL foreign key constraint is used when two tables in a database have a parent-child relationship and allows you to reference one table in a field of another table. The table containing the primary key should be defined before the child table.
The syntax for creating foreign keys in MySQL is Foreign Key (field_name) references parent_table_name (primary_key_name).
However, we need to keep in mind that the default MySQL database engine doesn?t support foreign keys; the InnoDB engine provides this functionality.
If you want to find out more about this topic, check out our article on MySQL’s foreign key constraint.
2. How Do You Read the MySQL Slow Query Log?
Assuming the slow query log is enabled, I prefer to use an analysis tool like mysqldumpslow. This tool parses MySQL slow query log files and only prints a summary. For a more detailed answer, see our detailed answer to this MySQL interview question.
3. How Do You Connect a MySQL Database to a HTML Web Page?
One of the most common MySQL interview questions ? you will almost definitely need this for a database management job.
- First, you need to gather the server name and the database login details, as well as the name of the table from which you want to display data on your HTML page.
- Then, create a .php file out of the table and add the php open and close tags before the HTML code. If opening the file in a web browser, it should only display the title tag.
- After this, perform a query within the body of the HTML page to read all the fields from the table.
- Then, add the data to the HTML page and close off the connection by adding the open and close PHP tags right after closing the HTML tag.
4. What Are Your Favorite DML Commands in MySQL and What Do They Do?
My favorite DML commands are the most straight-forward ones:
- SELECT: removes rows from a table with the syntax SELECT (column_name) from (table_name) where (conditions).
- UPDATE: Changes data within an existing entry in the database with the syntax UPDATE (table_name) SET (column_name=value) where (condition).
- INSERT: Adds records to a table in the database with the syntax INSERT INTO (table_name) (column) VALUES (value).
- DELETE: removes records from a table with the syntax DELETE FROM (table_name) where (condition).
5. What Are the Best MySQL Storage Engines?
One of the trickiest MySQL interview questions since the answer will almost always be subjectively analyzed, but here are some almost failproof options, depending on the purpose of the database in question.
- MyISAM storage engine: supported by all MySQL configurations and comes with extensions and features like B-tree indexes backup/point-in-time recovery, compression, encryption, geospatial data, indexing support, and many, many others.
- Memory storage engine: previously known as HEAP, this storage engine creates purpose tables for data that is stored in the computer memory. Comes with features such as encryption, pint-on-time recovery, hash indexes, update statistics, and many more.
- InnoDB storage engine: Default storage engine for MySQL 5.5. It has plenty of data protection capabilities using the foreign-key referential integrity constraints, as it follows the ACID model. It also comes with features like row-level locking.
- CSV storage engine: Stores the data as comma separated values instead of the standard table format, then compiles the engine in the MySQL server. It can be read and written with applications like Microsoft Excel, but it might not allow indexing, partitioning, or transactions.
6. How Do You Calculate the MySQL Date Difference?
This is one of the trickiest MySQL interview questions on this list. Entering the TIMESTAMPDIFF() function opens up a set of parenthesis in which you?ll have to set 3 parameters: unit of time, start date, and end date. If the unit of time is months, this function will automatically compensate for the number of days in each month.
Precision calculations require the use of the DATEDIFF() function after the TIMESTAMPDIFF() function. The parameters of the DATEDIFF() function need to be end date, start date + result of the previous TIMESTAMPDIFF() function. Then, divide the result of the first DATEDIFF() function by a second DATEDIFF() function that has start date + TIMESTAMPDIFF result + 1 month, then start date + TIMESTAMPDIFF().
7. What?s the Difference between SQL, MySQL, and SQL Server?
This is one of the first MySQL interview questions you will encounter in a MySQL interview. SQL stands for Standard Query Language, while MySQL and SQL Server are database management systems. SQL is used to interact with the data in databases.
MySQL is targeted towards internet servers and open-source software, while SQL server works better for corporate and enterprise markets. If you want speed and efficiency, MySQL is the way to go, as SQL Server is ideal for graphical data modelling and thorough administration.
8. What Are Some Advantages of MySQL?
First of all, MySQL is open-source and highly accessible as it?s very easy to install ? it takes minutes to download, install, and run it ? and requires little resources. It?s also very fast when compared to other database management systems and one of the most secure, too. I really like that it also comes with a graphic user interface as it makes it much easier to interact with the data.
9. What Are Some Disadvantages of MySQL?
This is one of the trickiest MySQL interview questions as many interviewees tend to focus on opinions rather than facts, but here’s a straight-forward answer. Although I appreciate the accessibility and speed it offers, MySQL has its faults. For once, it has incomplete capabilities as it?s not FULLY compliant with the SQL standard; it only implements a set of the standard. In addition, because it?s open-source, development might be slower than with others.
10. What Are the Main Differences Between HAVING and WHERE Clauses in MySQL?
Although these can be used interchangeably in some cases, there are a few things that differentiate the two. You can find the short answer to this MySQL interview question below; for the long answer, see our guide on the main differences between HAVING and WHERE.
First of all, the HAVING clause can?t be used with the GROUP BY clause ? this is when the WHERE clause comes in. Secondly, unlike the HAVING clause, the WHERE clause includes specific criteria records must follow to be selected by the query.
In addition, WHERE selects rows before grouping, while HAVING selects them after grouping. And lastly, the HAVING function can include aggregate functions while WHERE does not allow this.
11. What Is a Simple Key in MySQL?
Unlike primary, secondary, and foreign keys, simple keys have one single attribute and are used to identify a single entity.
Find out more about simple keys in database management systems here.
12. What Does Self JOIN Do and When Is It Useful?
Self JOIN essentially joins a table with itself, creating a unary relationship and joining each row of the table with itself, as well as the other rows in the table. The foreign key of the table will then reference its own primary key. Check out our detailed answer linked at the beginning of this paragraph for more info.
13. How to Use DISTINCT in MySQL?
DISTINCT retrieves values from a table and eliminates duplicates, while SELECT DISTINCT only retrieves values from the selected table.
The syntax is SELECT DISTINCT <column_name> from <table>;
To learn more about the DISTINCT statement in SQL-based database management systems, check out our article on DISTINCT in SQL.
14. What Is a Key in MySQL?
This is should be one of the easiest MySQL interview questions for anyone who wants to work as a MySQL developer. A key is a subclass of columns that?s been integrated into a table to allow the row to be uniquely identified. Each row must have a unique value for the key. There are several types of MySQL keys:
- Super Keys: unique field combinations that can uniquely identify a record from the table.
- Candidate Keys: consist of the minimum combination of fields that can uniquely identify a record. All tables should have at least one.
- Primary Keys: set of one or more fields or columns that uniquely identify an entry in the database.
- Foreign Keys: generate a certain relationship between two tables and can also be null or duplicate among different entries.
- Secondary Keys (aka alternate keys): can work as primary keys but are not defined as such. They?re basically spare primary keys.
- Composite Keys (aka compound keys): combination of multiple attributes that, together, uniquely identify an entry. These can become candidate or primary keys.
- Surrogate Keys: sequential numbers used to uniquely identify an entry and are only used as primary keys.
- Natural Keys (aka business keys or domain keys): composed of columns that have a logical relationship to other columns.
15. How Many Types of Tables Can You Have in MySQL?
The default table type is MyISAM and is based on the sequential access method, but we can also have:
- HEAP tables: ideal for fast data access, not ideal for security as all data is lost if the system crashes.
- InoDB tables: support transactions that require the COMMIT or ROLL BACK commands.
- BDB tables: similar to InoDB, but with?slower execution.
16. How Would You Select All Users Who Didn?t Leave a Phone Number when Registering?
This is one of our favorite MySQL interview questions on this list because it requires a straight-forward answer that can be objectively analyzed.
Assuming the table name as users, the column with their names as name, and the column with the phone number as phone_number, the command would look something like this:
SELECT name FROM users WHERE
17. What?s the Difference Between BLOB and TEXT?
The main difference between BLOB and TEXT is that BLOB is case sensitive when it comes to sorting and comparing entries, while TEXT is case insensitive. If this short answer leads to more MySQL interview questions from your interviewer, here’s some more info:
A BLOB is a large binary object that holds data. There are four types of BLOBs that only differ in the maximum length of the values they can hold and these are TINYBLOB, BLOB, MEDIUMBLOB, and LONGBLOB. The four types of TEXT correspond to the types of BLOB and have the same length and storage requirements: TINYTEXT, TEXT, MEDIUMTEXT, LONGTEXT.
18. What Are Access Control Lists & How Are They Used?
ACLs (Access Control Lists) are lists of objects and their associated permissions. These are stored in the cached memory and are automatically checked every time a user logs in or runs a command. It?s also used to troubleshoot any errors or issues users have with logging in or commands.
19. How Do You Use SAVEPOINT?
Savepoints are defined points in a transaction. They can be referenced in the ROLLBACK TO SAVEPOINT statement to roll back a transaction to a certain step along the way instead of undoing it completely. Therefore, the ROLLBACK TO SAVEPOINT savepointname command only rolls back the operations made after savepointname has been created. This is one of the most common MySQL interview questions for experienced candidates.
20. Can You Enter HEX Numbers in a MySQL Database?
Yes, you can. You just have to enter the regular HEX numbers with single quotes and X as a prefix or simply prefix them with 0x, like so: X?01AF? or 0x01AF. These prefixes are case sensitive, which means x?01AF?, 0X01AF, and anything else that doesn?t follow the format in bold is deemed as an illegal hex literal and will not be processed correctly.
Even though MySQL is one of the most straight-forward database management systems, it still raises difficulties for experienced and beginner programmers and database administrators alike. Hopefully, these MySQL interview questions and answers cleared up at least some of the concepts we covered. Alternatively, you can also check out our collection of SQL interview questions. These might give you some insight on database management and help you grasp concepts a little better.
Do you have other challenging database interview questions you came across when interviewing for a MySQL position?