SQL interview questions by jeetendra mandal - part 3
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A database is an organized collection of structured data stored digitally. A DBMS is a system software that creates, retrieves, updates and manages databases, serving as an interface between databases and users. An RDBMS stores data in tables with defined relations between common fields of different tables. SQL is the standard language used to retrieve and manipulate structured data in relational databases, while MySQL is an RDBMS that manages SQL databases.
SQL interview questions by jeetendra mandal - part 3
- 2. 1. What is Database? A database is an organized collection of data, stored and retrieved digitally from a remote or local computer system. Databases can be vast and complex, and such databases are developed using fixed design and modeling approaches. 2. What is DBMS? DBMS stands for Database Management System. DBMS is a system software responsible for the creation, retrieval, updation, and management of the database. It ensures that our data is consistent, organized, and is easily accessible by serving as an interface between the database and its end-users or application software. 3. What is RDBMS? How is it different from DBMS? RDBMS stands for Relational Database Management System. The key difference here, compared to DBMS, is that RDBMS stores data in the form of a collection of tables, and relations can be defined between the common fields of these tables. Most modern database management systems like MySQL, Microsoft SQL Server, Oracle, IBM DB2, and Amazon Redshift are based on RDBMS.
- 3. 4. What is SQL? SQL stands for Structured Query Language. It is the standard language for relational database management systems. It is especially useful in handling organized data comprised of entities (variables) and relations between different entities of the data. 5. What is the difference between SQL and MySQL? SQL is a standard language for retrieving and manipulating structured databases. On the contrary, MySQL is a relational database management system, like SQL Server, Oracle or IBM DB2, that is used to manage SQL databases.
- 4. 6. What are Tables and Fields? A table is an organized collection of data stored in the form of rows and columns. Columns can be categorized as vertical and rows as horizontal. The columns in a table are called fields while the rows can be referred to as records. 7. What are Constraints in SQL? Constraints are used to specify the rules concerning data in the table. It can be applied for single or multiple fields in an SQL table during the creation of the table or after creating using the ALTER TABLE command. The constraints are: NOT NULL - Restricts NULL value from being inserted into a column. CHECK - Verifies that all values in a field satisfy a condition. DEFAULT - Automatically assigns a default value if no value has been specified for the field. UNIQUE - Ensures unique values to be inserted into the field. INDEX - Indexes a field providing faster retrieval of records. PRIMARY KEY - Uniquely identifies each record in a table. FOREIGN KEY - Ensures referential integrity for a record in another table.
- 5. 8. What is a Primary Key? The PRIMARY KEY constraint uniquely identifies each row in a table. It must contain UNIQUE values and has an implicit NOT NULL constraint. A table in SQL is strictly restricted to have one and only one primary key, which is comprised of single or multiple fields (columns). 9. How to delete a table in SQL? There are two ways to delete a table from sql: DROP and TRUNCATE. The DROP TABLE command is used to completely delete the table from the database. This is the command: DROP TABLE table_name; The above command will completely delete all the data present in the table along with the table itself. But if we want to delete only the data present in the table but not the table itself, then we will use the truncate command: DROP TABLE table_name ;
- 6. 10. How to change a table name in SQL? This is the command to change a table name in SQL: ALTER TABLE table_name RENAME TO new_table_name; We will start off by giving the keywords ALTER TABLE, then we will follow it up by giving the original name of the table, after that, we will give in the keywords RENAME TO and finally, we will give the new table name. For example, if we want to change the “employee” table to “employee_information”, this will be the command: ALTER TABLE employee RENAME TO employee_information;