Mysql Queries Interview Questions And Answers
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Mysql Queries Interview Questions And Answers
MySQL queries are questions or commands used to retrieve and manipulate data from a MySQL database. These queries are written in Structured Query Language (SQL) and are used to perform various tasks such as selecting, inserting, updating, and deleting data from database tables. In an interview, candidates may be asked to write and explain different types of MySQL queries, such as SELECT, INSERT, UPDATE, and DELETE, and demonstrate their understanding of database concepts like table relationships, indexes, and joins. It is important for candidates to have a strong understanding of SQL syntax and the MySQL database structure to effectively answer these questions. Additionally, they should be able to troubleshoot and optimize queries to improve database performance.
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1 - What is a SQL query?
A SQL query is a request made to a database management system (DBMS) for specific information from the database. It is used to retrieve, update, insert or delete data from a database according to certain conditions.
2) What is the difference between a join and a subquery?
A join combines data from multiple tables into a single result set, while a subquery is a nested query within another query used to filter or manipulate the data.
3) How can you optimize a SQL query?
Some ways to optimize a SQL query include using appropriate indexes, minimizing the use of select * to only necessary columns, avoiding using functions in the WHERE clause, and using EXPLAIN to analyze the query execution plan.
4) What is the purpose of using the GROUP BY clause in a SQL query?
The GROUP BY clause is used to group the result set by one or more columns. It is typically used with aggregate functions like SUM, AVG, COUNT to perform calculations on data within each group.
5) What are the different types of SQL joins?
The different types of SQL joins are INNER JOIN (also known as EQUIJOIN), LEFT OUTER JOIN, RIGHT OUTER JOIN, and FULL OUTER JOIN.
6) How can you retrieve the first n records from a table in SQL?
To retrieve the first n records, you can use the TOP or LIMIT clause in SQL, depending on the database management system being used.
7) What is a subquery?
A subquery is a nested query within another query used to filter or manipulate data. It is enclosed within parentheses and can be used in the SELECT, FROM, WHERE, or HAVING clause.
8) What is a SQL view?
A SQL view is a virtual table created from a query. It does not store data itself, but acts as a virtual table that can be queried like a regular table.
9) How can you identify duplicate records in a table?
To identify duplicate records, you can use the GROUP BY clause along with the COUNT() function to count the number of occurrences for each group. Records with a count greater than 1 are duplicates.
10) What is the difference between DELETE and TRUNCATE statements in SQL?
DELETE is a DML (Data Manipulation Language) statement used to remove one or more records from a table, with the option of using a WHERE clause. TRUNCATE is a DDL (Data Definition Language) statement used to remove all rows from a table without any conditions.
11) How can you add a new column to a table in SQL?
You can add a new column using the ALTER TABLE statement, specifying the table name, the new column name, and its data type.
12) What is a primary key?
A primary key is a column or a combination of columns that uniquely identifies each row in a table. It is used to enforce data integrity and ensure the uniqueness of records.
13) How can you sort the result set in a specific order in SQL?
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To sort the result set, you can use the ORDER BY clause, specifying the column name and the sorting direction (ASC for ascending, DESC for descending).
14) How do you handle NULL values in SQL?
NULL values can be handled by using functions like IS NULL, IS NOT NULL or COALESCE to check for the presence of NULL values and replace them with a default value.
15) What is a stored procedure?
A stored procedure is a set of SQL statements that are stored in the database and can be executed by calling the procedure instead of writing the individual SQL statements. It can also accept input parameters and return output parameters.
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