SQL Interview Questions and Answers for Beginners - Comprehensive Guide - 常规

What is SQL?\nSQL stands for Structured Query Language, which is a programming language used for managing and manipulating relational databases. It is used to create, modify, and retrieve data from databases.\n\n2. What are the different types of SQL statements?\nThere are mainly four types of SQL statements:\n- DDL (Data Definition Language): Used to define and manage the structure of the database, such as creating tables, altering table structures, etc.\n- DML (Data Manipulation Language): Used to manipulate data in the database, such as inserting, updating, deleting records.\n- DQL (Data Query Language): Used to retrieve data from the database, such as SELECT statements.\n- DCL (Data Control Language): Used to control access to the database, such as granting and revoking permissions.\n\n3. What is a primary key?\nA primary key is a column or a set of columns in a table that uniquely identifies each row in the table. It must have a unique value for each row and cannot contain null values.\n\n4. What is a foreign key?\nA foreign key is a column or a set of columns in a table that refers to the primary key of another table. It establishes a relationship between two tables and ensures referential integrity.\n\n5. What is normalization in SQL?\nNormalization is the process of organizing data in a database to efficiently eliminate redundancy and improve data integrity. It involves dividing a database into two or more tables and defining relationships between them.\n\n6. What is the difference between UNION and UNION ALL in SQL?\nUNION is used to combine the result sets of two or more SELECT statements, removing duplicate rows. UNION ALL, on the other hand, also combines the result sets but does not remove duplicates.\n\n7. What is the difference between DELETE and TRUNCATE in SQL?\nDELETE is a DML statement used to delete specific rows from a table based on a condition. It generates a log entry for each deleted row and can be rolled back.\nTRUNCATE is a DDL statement used to remove all rows from a table. It does not generate log entries and cannot be rolled back.\n\n8. What is the difference between INNER JOIN and OUTER JOIN?\nINNER JOIN returns only the rows that have matching values in both tables being joined. It discards non-matching rows from both tables.\nOUTER JOIN returns all the rows from one table and the matching rows from the other table. It includes non-matching rows and fills the missing values with NULL.\n\n9. What is a stored procedure?\nA stored procedure is a set of SQL statements that are precompiled and stored in a database. It can be called and executed multiple times. It is used for encapsulating and executing complex logic on the database server.\n\n10. What is a view in SQL?\nA view is a virtual table derived from one or more tables. It does not store any data itself but retrieves data from the underlying tables based on the query defined while creating the view. Views are used for simplifying complex queries and providing a layer of abstraction.