SQL Basic Commands & Interview Questions

SELECT * FROM Customers;

SELECT CustomerName, City FROM Customers;

SELECT DISTINCT Country FROM Customers;

SELECT * FROM Customers  WHERE Country='Mexico';

SELECT * FROM Products  ORDER BY Price;

SELECT * FROM Products ORDER BY Price DESC;

SELECT * FROM Customers ORDER BY Country, CustomerName;

SELECT * FROM Customers ORDER BY Country ASC, CustomerName DESC;

SELECT * FROM Customers WHERE Country = 'Spain' AND CustomerName LIKE 'G%';

SELECT * FROM Customers WHERE Country = 'Germany' OR Country = 'Spain';

SELECT * FROM Customers 

WHERE Country = 'Spain' AND (CustomerName LIKE 'G%' OR CustomerName LIKE 'R%');

SELECT * FROM Customers
WHERE Country = 'Spain' AND CustomerName LIKE 'G%' OR CustomerName LIKE 'R%'; 

SELECT * FROM Customers WHERE NOT Country = 'Spain';

SELECT * FROM Customers WHERE CustomerName NOT LIKE 'A%';

SELECT * FROM Customers WHERE CustomerID NOT BETWEEN 10 AND 60;

SELECT * FROM Customers WHERE City NOT IN ('Paris', 'London');

INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country)
VALUES ('Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway');

INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country)
VALUES
('Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway'),
('Greasy Burger', 'Per Olsen', 'Gateveien 15', 'Sandnes', '4306', 'Norway'),
('Tasty Tee', 'Finn Egan', 'Streetroad 19B', 'Liverpool', 'L1 0AA', 'UK');

SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NULL;

SELECT CustomerName, ContactName, Address FROM Customers WHERE Address IS NOT NULL;

UPDATE Customers SET ContactName = 'Alfred Schmidt', City= 'Frankfurt' WHERE CustomerID = 1;

DELETE FROM Customers WHERE CustomerName='Alfreds Futterkiste';

DELETE FROM Customers;

DROP TABLE Customers;

SELECT TOP 3 * FROM Customers;

SELECT * FROM Customers LIMIT 3;

SELECT * FROM Customers FETCH FIRST 3 ROWS ONLY;

SELECT TOP 50 PERCENT * FROM Customers;

SELECT TOP 3 * FROM Customers ORDER BY CustomerName DESC;

SELECT MIN(Price) FROM Products;

SELECT MAX(Price) FROM Products;

SELECT MIN(Price) AS SmallestPrice FROM Products;

SELECT MIN(Price) AS SmallestPrice, CategoryID FROM Products GROUP BY CategoryID;

SELECT COUNT(DISTINCT Price) FROM Products;

SELECT SUM(Quantity) FROM OrderDetails WHERE ProductId = 11;

SELECT OrderID, SUM(Quantity) AS [Total Quantity] FROM OrderDetails GROUP BY OrderID;

SELECT SUM(Quantity * 10) FROM OrderDetails;

SELECT SUM(Price * Quantity) FROM OrderDetails

LEFT JOIN Products ON OrderDetails.ProductID = Products.ProductID;

SELECT * FROM Products WHERE price > (SELECT AVG(price) FROM Products);

SELECT * FROM Customers WHERE city LIKE 'L_nd__';

SELECT * FROM Customers WHERE city LIKE '%L%';

SELECT * FROM Customers WHERE CustomerName LIKE 'b%s'; 

SELECT * FROM Customers WHERE CustomerName LIKE '_r%';

SELECT * FROM Customers WHERE CustomerName LIKE '[bsp]%';

SELECT * FROM Customers WHERE CustomerName LIKE '[a-f]%';

SELECT * FROM Customers WHERE Country IN ('Germany', 'France', 'UK');

SELECT * FROM Products WHERE Price BETWEEN 10 AND 20;

SELECT * FROM Products WHERE Price BETWEEN 10 AND 20 AND CategoryID IN (1,2,3);

SELECT CustomerID AS ID FROM Customers;

SELECT CustomerID AS ID, CustomerName AS Customer FROM Customers;

SELECT ProductName AS [My Great Products] FROM Products;

SELECT CustomerName, Address + ', ' + PostalCode + ' ' + City + ', ' + Country AS Address 

FROM Customers;

SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;

SELECT ProductID, ProductName, CategoryName FROM Products
INNER JOIN Categories ON Products.CategoryID = Categories.CategoryID;

SELECT Customers.CustomerName, Orders.OrderID FROM Customers
LEFT JOIN Orders ON Customers.CustomerID = Orders.CustomerID
ORDER BY Customers.CustomerName;

SELECT Orders.OrderID, Employees.LastName, Employees.FirstName FROM Orders
RIGHT JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID
ORDER BY Orders.OrderID;

SELECT Customers.CustomerName, Orders.OrderID FROM Customers
FULL OUTER JOIN Orders ON Customers.CustomerID=Orders.CustomerID
ORDER BY Customers.CustomerName;

SELECT A.CustomerName AS CustomerName1, B.CustomerName AS CustomerName2, A.City
FROM Customers A, Customers B
WHERE A.CustomerID <> B.CustomerID AND A.City = B.City ORDER BY A.City;

SELECT City FROM Customers UNION SELECT City FROM Suppliers ORDER BY City;

SELECT City FROM Customers UNION ALL SELECT City FROM Suppliers ORDER BY City;

SELECT COUNT(CustomerID), Country FROM Customers GROUP BY Country;

SELECT Shippers.ShipperName, COUNT(Orders.OrderID) AS NumberOfOrders FROM Orders
LEFT JOIN Shippers ON Orders.ShipperID = Shippers.ShipperID GROUP BY ShipperName;

SELECT COUNT(CustomerID), Country FROM Customers GROUP BY Country 

HAVING COUNT(CustomerID) > 5;

SELECT Employees.LastName, COUNT(Orders.OrderID) AS NumberOfOrders FROM (Orders
INNER JOIN Employees ON Orders.EmployeeID = Employees.EmployeeID) GROUP BY LastName
HAVING COUNT(Orders.OrderID) > 10;

SELECT SupplierName FROM Suppliers
WHERE EXISTS (SELECT ProductName FROM Products WHERE Products.SupplierID = Suppliers.supplierID AND Price < 20);

SELECT ProductName FROM Products WHERE ProductID = ANY
  (SELECT ProductID FROM OrderDetails WHERE Quantity = 10);

SELECT * INTO CustomersBackup2017 FROM Customers;

SELECT CustomerName, ContactName INTO CustomersBackup2017 FROM Customers;

INSERT INTO Customers (CustomerName, City, Country)
SELECT SupplierName, City, Country FROM Suppliers;

INSERT INTO Customers (CustomerName, City, Country)
SELECT SupplierName, City, Country FROM Suppliers WHERE Country='Germany';

SELECT OrderID, Quantity,
CASE
    WHEN Quantity > 30 THEN 'The quantity is greater than 30'
    WHEN Quantity = 30 THEN 'The quantity is 30'
    ELSE 'The quantity is under 30'
END AS QuantityText FROM OrderDetails;

CREATE PROCEDURE SelectAllCustomers AS SELECT * FROM Customers GO;

EXEC SelectAllCustomers; 

SELECT * FROM Customers -- WHERE City='Berlin';  

What is SQL?

It stands for Structured Query Language. A programming language used for interaction with relational database management systems (RDBMS). This includes fetching, updating, inserting, and removing data from tables.

2. What are SQL dialects? Give some examples.

The various versions of SQL, both free and paid, are also called SQL dialects. All the flavours of SQL have a very similar syntax and vary insignificantly only in additional functionality. Some examples are Microsoft SQL Server, PostgreSQL, MySQL, SQLite, T-SQL, Oracle, and MongoDB.

3. What are the main applications of SQL?

Using SQL, we can:

• create, delete, and update tables in a database
• access, manipulate, and modify data in a table
• retrieve and summarise the necessary information from a table or several tables
• add or remove certain rows or columns from a table

All in all, SQL allows querying a database in multiple ways. In addition, SQL easily integrates with other programming languages, such as Python or R, so we can use their combined power.

4. What is an SQL statement? Give some examples.

Also known as an SQL command. It's a string of characters interpreted by the SQL engine as a legal command and executed accordingly. Some examples of SQL statements are SELECT, CREATE, DELETE, DROP, REVOKE, and so on.

5. What types of SQL commands (or SQL subsets) do you know?

• Data Definition Language (DDL) – to define and modify the structure of a database.
• Data Manipulation Language (DML) – to access, manipulate, and modify data in a database.
• Data Control Language (DCL) – to control user access to the data in the database and give or revoke privileges to a specific user or a group of users.
• Transaction Control Language (TCL) – to control transactions in a database.
• Data Query Language (DQL) – to perform queries on the data in a database to retrieve the necessary information from it.

6. Give some examples of common SQL commands of each type.

• DDL: CREATE, ALTER TABLE, DROP, TRUNCATE, and ADD COLUMN
• DML: UPDATE, DELETE, and INSERT
• DCL: GRANT and REVOKE
• TCL: COMMIT, SET TRANSACTION, ROLLBACK, and SAVEPOINT
• DQL: – SELECT

7. What is a database?

A structured storage space where the data is kept in many tables and organized so that the necessary information can be easily fetched, manipulated, and summarized.

8. What is DBMS, and what types of DBMS do you know?

It stands for Database Management System, a software package used to perform various operations on the data stored in a database, such as accessing, updating, wrangling, inserting, and removing data. There are various types of DBMS, such as relational, hierarchical, network, graph, or object-oriented. These types are based on the way the data is organised, structured, and stored in the system.

9. What is RDBMS? Give some examples of RDBMS.

It stands for Relational Database Management System. It's the most common type of DBMS used for working with data stored in multiple tables related to each other by means of shared keys. The SQL programming language is particularly designed to interact with RDBMS. Some examples of RDBMS are MySQL, PostgreSQL, Oracle, MariaDB, etc.

10. What are tables and fields in SQL?

A table is an organised set of related data stored in a tabular form, i.e., in rows and columns. A field is another term for a column of a table.

11. What is an SQL query, and what types of queries do you know?

A query is a piece of code written in SQL to access the data from a database or to modify the data. Correspondingly, there are two types of SQL queries: select and action queries. The first ones are used to retrieve the necessary data (this also includes limiting, grouping, ordering the data, extracting the data from multiple tables, etc.), while the second ones are used to create, add, delete, update, rename the data, etc.

12. What is a subquery?

Also called an inner query; a query placed inside another query, or an outer query. A subquery may occur in the clauses such as SELECT, FROM, WHERE, UPDATE, etc. It's also possible to have a subquery inside another subquery. The innermost subquery is run first, and its result is passed to the containing query (or subquery).

13. What types of SQL subqueries do you know?

• Single-row – returns at most one row.
• Multi-row – returns at least two rows.
• Multi-column – returns at least two columns.
• Correlated – a subquery related to the information from the outer query.
• Nested – a subquery inside another subquery.

14. What is a constraint, and why use constraints?

A set of conditions defining the type of data that can be input into each column of a table. Constraints ensure data integrity in a table and block undesired actions.

15. What SQL constraints do you know?

• DEFAULT – provides a default value for a column.
• UNIQUE – allows only unique values.
• NOT NULL – allows only non-null values.
• PRIMARY KEY – allows only unique and strictly non-null values (NOT NULL and UNIQUE).
• FOREIGN KEY – provides shared keys between two and more tables.

16. What is a join?

A clause used to combine and retrieve records from two or multiple tables. SQL tables can be joined based on the relationship between the columns of those tables. Check out our SQL joins tutorial for more context. 

17. What types of joins do you know?

• (INNER) JOIN – returns only those records that satisfy a defined join condition in both (or all) tables. It's a default SQL join.
• LEFT (OUTER) JOIN – returns all records from the left table and those records from the right table that satisfy a defined join condition.
• RIGHT (OUTER) JOIN – returns all records from the right table and those records from the left table that satisfy a defined join condition.
• FULL (OUTER) JOIN – returns all records from both (or all) tables. It can be considered as a combination of left and right joins.

18. What is a primary key?

A column (or multiple columns) of a table to which the PRIMARY KEY constraint was imposed to ensure unique and non-null values in that column. In other words, a primary key is a combination of the NOT NULL and UNIQUE constraints. The primary key uniquely identifies each record of the table. Each table should contain a primary key and can't contain more than one primary key.

19. What is a unique key?

A column (or multiple columns) of a table to which the UNIQUE constraint was imposed to ensure unique values in that column, including a possible NULL value (the only one).

20. What is a foreign key?

A column (or multiple columns) of a table to which the FOREIGN KEY constraint was imposed to link this column to the primary key in another table (or several tables). The purpose of foreign keys is to keep connected various tables of a database.

21. What is an index?

A special data structure related to a database table and used for storing its important parts and enabling faster data search and retrieval. Indexes are especially efficient for large databases, where they significantly enhance query performance.

22. What types of indexes do you know?

• Unique index – doesn't allow duplicates in a table column and hence helps maintain data integrity.
• Clustered index – defines the physical order of records of a database table and performs data searching based on the key values. A table can have only one clustered index.
• Non-clustered index – keeps the order of the table records that doesn't match the physical order of the actual data on the disk. It means that the data is stored in one place and a non-clustered index – in another one. A table can have multiple non-clustered indexes.

23. What is a schema?

A collection of database structural elements such as tables, stored procedures, indexes, functions, and triggers. It shows the overall database architecture, specifies the relationships between various objects of a database, and defines different access permissions for them.

24. What is a SQL comment?

A human-readable clarification on what a particular piece of code does. SQL code comments can be single-line (preceded by a double dash --) or span over multiple lines (as follows: /*comment_text*/). When the SQL engine runs, it ignores code comments. The purpose of adding SQL code comments is to make the code more comprehensive for those people who will read it in the future.

25. What is a SQL operator?

A reserved character, a combination of characters, or a keyword used in SQL queries to perform a specific operation. SQL operators are commonly used with the WHERE clause to set a condition (or conditions) for filtering the data.

26. What types of SQL operators do you know?

• Arithmetic (+, -, *, /, etc.)
• Comparison (>, <, =, >=, etc.)
• Compound (+=, -=, *=, /=, etc.)
• Logical (AND, OR, NOT, BETWEEN, etc.)
• String (%, _, +, ^, etc.)
• Set (UNION, UNION ALL, INTERSECT, and MINUS (or EXCEPT))

27. What is an alias?

A temporary name given to a table (or a column in a table) while executing a certain SQL query. Aliases are used to improve the code readability and make the code more compact. An alias is introduced with the AS keyword:

SELECT col_1 AS column
FROM table_name;

28. What is a clause?

A condition imposed on a SQL query to filter the data to obtain the desired result. Some examples are WHERE, LIMIT, HAVING, LIKE, AND, OR, ORDER BY, etc.

29. What are some common statements used with the SELECT query?

The most common ones are FROM, GROUP BY, JOIN, WHERE, ORDER BY, LIMIT, and HAVING.

30. How to create a table?

Using the CREATE TABLE statement. For example, to create a table with three columns of predefined datatypes, we apply the following syntax:

CREATE TABLE table_name (col_1 datatype,
                         col_2 datatype,
                         col_3 datatype);

31. How to update a table?

Using the UPDATE statement. The syntax is:

UPDATE table_name
SET col_1 = value_1, column_2 = value_2
WHERE condition;

32. How to delete a table from a database?

Using the DROP TABLE statement. The syntax is: DROP TABLE table_name;.

33. How to get the count of records in a table?

Using the COUNT() aggregate function with the asterisk passed as its argument: SELECT COUNT(*) FROM table_name;.

34. How to sort records in a table?

Using the ORDER BY statement:

SELECT * FROM table_name
ORDER BY col_1;

We can specify that we need a descending order using the DESC keyword; otherwise, the order will be ascending by default. Also, we can sort by more than one column and specify for each one, ascending or descending order separately. For example:

SELECT * FROM table_name
ORDER BY col_1 DESC, col_3, col_6 DESC;

35. How to select all columns from a table?

Using the asterisk * with the SELECT statement. The syntax is: SELECT * FROM table_name;.

36. How to select common records from two tables?

Using the INTERSECT statement:
SELECT * FROM table_1
INTERSECT
SELECT * FROM table_1;

37. What is the DISTINCT statement and how do you use it?

This statement is used with the SELECT statement to filter out duplicates and return only unique values from a column of a table. The syntax is:

SELECT DISTINCT col_1
FROM table_name;

38. What are entities? Give some examples.

An entity is a real-world object, creature, place, or phenomenon for which the data can be gathered and stored in a database table. Each entity corresponds to a row in a table, while the table's columns describe its properties. Some examples of entities are bank transactions, students in a school, cars sold, etc.

39. What are relationships? Give some examples.

Relationships are the connections and correlations between entities, basically meaning how two or more tables of a database are related to one another. For example, we can find an ID of the same client in a table on sales data and in a customer table.

40. What is NULL value? How is it different from zero or a blank space?

A NULL value indicates the absence of data for a certain cell of a table. Instead, zero is a valid numeric value, and an empty string is a legal string of zero length.

Intermediate SQL Interview Questions and Answers

In this section, we take a look at the 40 most popular intermediate SQL questions and answers, so that you'll know what to expect from your interviewer. These questions are more suited to SQL practitioners with a few years of experience. 

1. What is a function in SQL, and why use functions?

A database object representing a set of SQL statements frequently used for a certain task. A function takes in some input parameters, performs calculations or other manipulations on them, and returns the result. Functions help improve code readability and avoid repetition of the same code snippets.

2. What types of SQL functions do you know?

• Aggregate functions – work on multiple, usually grouped records for the provided columns of a table, and return a single value (usually by group).
• Scalar functions – work on each individual value and return a single value.

On the other hand, SQL functions can be built-in (defined by the system) or user-defined (created by the user for their specific needs).

3. What aggregate functions do you know?

• AVG() – returns the average value
• SUM() – returns the sum of values
• MIN() – returns the minimum value
• MAX() – returns the maximum value
• COUNT() – returns the number of rows, including those with null values
• FIRST() – returns the first value from a column
• LAST()– returns the last value from a column

4. What scalar functions do you know?

• LEN() (in other SQL flavors – LENGTH()) – returns the length of a string, including the blank spaces
• UCASE() (in other SQL flavors – UPPER()) – returns a string converted to the upper case
• LCASE() (in other SQL flavors – LOWER()) – returns a string converted to the lower case
• INITCAP() – returns a string converted to the title case (i.e., each word of the string starts from a capital letter)
• MID() (in other SQL flavors – SUBSTR()) – extracts a substring from a string
• ROUND() – returns the numerical value rounded to a specified number of decimals
• NOW() – returns the current date and time

5. What are case manipulation functions? Give some examples.

Case manipulation functions represent a subset of character functions, and they're used to change the case of the text data. With these functions, we can convert the data into the upper, lower, or title case.

• UCASE() (in other SQL flavors – UPPER()) – returns a string converted to the upper case
• LCASE() (in other SQL flavors – LOWER()) – returns a string converted to the lower case
• INITCAP() – returns a string converted to the title case (i.e., each word of the string starts from a capital letter)

6. What are character manipulation functions? Give some examples.

Character manipulation functions represent a subset of character functions, and they're used to modify the text data.

• CONCAT() – joins two or more string values appending the second string to the end of the first one
• SUBSTR() – returns a part of a string satisfying the provided start and end points
• LENGTH() (in other SQL flavors – LEN()) – returns the length of a string, including the blank spaces
• REPLACE() – replaces all occurrences of a defined substring in a provided string with another substring
• INSTR() – returns the numeric position of a defined substring in a provided string
• LPAD() and RPAD() – return the padding of the left-side/right-side character for right-justified/left-justified value
• TRIM() – removes all the defined characters, as well as white spaces, from the left, right, or both ends of a provided string

7. What is the difference between local and global variables?

Local variables can be accessed only inside the function in which they were declared. Instead, global variables, being declared outside any function, are stored in fixed memory structures and can be used throughout the entire program.

8. What is the default data ordering with the ORDER BY statement, and how do you change it?

By default, the order is ascending. To change it to descending, we need to add the DESC keyword as follows:

SELECT * FROM table_name
ORDER BY col_1 DESC;

9. What set operators do you know?

• UNION – returns the records obtained by at least one of two queries (excluding duplicates)
• UNION ALL – returns the records obtained by at least one of two queries (including duplicates)
• INTERSECT – returns the records obtained by both queries
• EXCEPT (called MINUS in MySQL and Oracle) – returns only the records obtained by the first query but not the second one

10. What operator is used in the query for pattern matching?

The LIKE operator in combination with the % and _ wildcards. The % wildcard represents any number of characters including zero, while _ – strictly one character.

11. What is the difference between a primary key and a unique key?

While both types of keys ensure unique values in a column of a table, the first one identifies uniquely each record of the table, and the second one prevents duplicates in that column.

12. What is a composite primary key?

The primary key of a table, based on multiple columns.

13. What is the order of appearance of the common statements in the SELECT query?

SELECT – FROM – JOIN – ON – WHERE – GROUP BY – HAVING – ORDER BY – LIMIT

14. In which order the interpreter executes the common statements in the SELECT query?

FROM – JOIN – ON – WHERE – GROUP BY – HAVING – SELECT – ORDER BY – LIMIT

15. What is a view, and why use it?

A virtual table containing a subset of data retrieved from one or more database tables (or other views). Views take very little space, simplify complex queries, limit access to the data for security reasons, enable data independence, and summarize data from multiple tables.

16. Can we create a view based on another view?

Yes. This is also known as nested views. However, we should avoid nesting multiple views since the code becomes difficult to read and debug.

17. Can we still use a view if the original table is deleted?

No. Any views based on that table will become invalid after deleting the base table. If we try to use such a view anyway, we'll receive an error message.

18. What types of SQL relationships do you know?

• One-to-one – each record in one table corresponds to only one record in another table
• One-to-many – each record in one table corresponds to several records in another table
• Many-to-many – each record in both tables corresponds to several records in another table

19. What are the possible values of a BOOLEAN data field?

In some SQL flavors, such as PostgreSQL, the BOOLEAN data type exists explicitly and takes values TRUE, FALSE, or NULL. In other flavors, such as Microsoft SQL Server, the BIT datatype is used to store Boolean values as integers 1 (true) or 0 (false).

20. What is normalization in SQL, and why use it?

Normalization is a process of database design that includes organizing and restructuring data in a way to reduce data redundancy, dependency, duplication, and inconsistency. This leads to enhanced data integrity, more tables within the database, more efficient data access and security control, and greater query flexibility.

21. What is denormalization in SQL, and why use it?

Denormalization is the process opposite of normalization: it introduces data redundancy and combines data from multiple tables. Denormalization optimizes the performance of the database infrastructure in situations when read operations are more important than write operations since it helps avoid complex joins and reduces the time of query running.

22. What is the difference between renaming a column and giving an alias to it?

Renaming a column means permanently changing its actual name in the original table. Giving an alias to a column means giving it a temporary name while executing an SQL query, with the purpose to make the code more readable and compact.

23. What is the difference between nested and correlated subqueries?

A correlated subquery is an inner query nested in a bigger (outer) query that refers to the values from the outer query for its execution, meaning that a correlated subquery depends on its outer query. Instead, a non-correlated subquery doesn't rely on the data from the outer query and can be run independently of it.

24. What is the difference between clustered and non-clustered indexes?

While a clustered index defines the physical order of records of a table and performs data searching based on the key values, a non-clustered index keeps the order of records that doesn't match the physical order of the actual data on the disk. A table can have only one clustered index but many non-clustered ones.

25. What is the CASE() function?

The way to implement the if-then-else logic in SQL. This function sequentially checks the provided conditions in the WHEN clauses and returns the value from the corresponding THEN clause when the first condition is satisfied. If none of the conditions is satisfied, the function returns the value from the ELSE clause in case it's provided, otherwise, it returns NULL. The syntax is:

CASE
    WHEN condition_1 THEN value_1
    WHEN condition_2 THEN value_2
    WHEN condition_3 THEN value_3
    ...
    ELSE value
END;

26. What is the difference between the DELETE and TRUNCATE statements?

DELETE is a reversible DML (Data Manipulation Language) command used to delete one or more rows from a table based on the conditions specified in the WHERE clause. Instead, TRUNCATE is an irreversible DDL (Data Definition Language) command used to delete all rows from a table. DELETE works slower than TRUNCATE. Also, we can't use the TRUNCATE statement for a table containing a foreign key.

27. What is the difference between the DROP and TRUNCATE statements?

DROP deletes a table from the database completely, including the table structure and all the associated constraints, relationships with other tables, and access privileges. TRUNCATE deletes all rows from a table without affecting the table structure and constraints. DROP works slower than TRUNCATE. Both are irreversible DDL (Data Definition Language) commands.

28. What is the difference between the HAVING and WHERE statements?

The first one works on aggregated data after they are grouped, while the second one checks each row individually. If both statements are present in a query, they appear in the following order: WHERE – GROUP BY – HAVING. The SQL engine interprets them also in the same order.

29. How do you add a record to a table?

Using the INSERT INTO statement in combination with VALUES. The syntax is:

INSERT INTO table_name
VALUES (value_1, value_2, ...);

30. How to delete a record from a table?

Using the DELETE statement. The syntax is:

DELETE FROM table_name
WHERE condition;

In this way, we can also delete multiple records if they satisfy the provided condition.

31. How to add a column to a table?

Using the ALTER TABLE statement in combination with ADD. The syntax is:

ALTER TABLE table_name
ADD column_name datatype;

32. How to rename a column of a table?

Using the ALTER TABLE statement in combination with RENAME COLUMN ... TO ... The syntax is:

ALTER TABLE table_name
RENAME COLUMN old_column_name TO new_column_name;

33. How to delete a column from a table?

Using the ALTER TABLE statement in combination with DROP COLUMN. The syntax is:

ALTER TABLE table_name
DROP COLUMN column_name;

34. How to select all even or all odd records in a table?

By checking the remainder of the division by 2. In some SQL versions (e.g., PostgreSQL and My SQL), we use the MOD function, in the others (Microsoft SQL Server and SQLite) – the modulo operator (%). To select all even records using MOD:

SELECT * FROM table_name
WHERE MOD(ID_column, 2) = 0;

To select all even records using %:

SELECT * FROM table_name 
WHERE ID_column % 2 = 0;

To select all odd records, the syntax is identical in both cases, only that we would use the inequality operator <> instead of =.

35. How to prevent duplicate records when making a query?

Using the DISTINCT statement in combination with SELECT or creating a unique key for that table.

36. How to insert many rows in a table?

Using the INSERT INTO statement in combination with VALUES. The syntax is:

INSERT INTO table_name
VALUES (value_1, value_2, ...),
      (value_3, value_4, ...),
      (value_5, value_6, ...),
      ...;

37. How to find the nth highest value in a column of a table?

Using the OFFSET clause. For example, to find the 6th highest value from a column, we would use the following syntax:

SELECT * FROM table_name
ORDER BY column_name DESC
LIMIT 1
OFFSET 5;

38. How to find the values in a text column of a table that start with a certain letter?

Using the LIKE operator in combination with the % and _ wildcards. For example, we need to find all surnames in a table that start with "A". The query is:

SELECT * FROM table_name
WHERE surname LIKE 'A_';

Here, we assume that a surname must contain at least two letters. Without this assumption (meaning that a surname can be just A), the query is as follows:

SELECT * FROM table_name
WHERE surname LIKE 'A%';

39. How to find the last id in a table?

Using the MAX() function. Otherwise, in many SQL versions, we can use the following syntax:

SELECT id
FROM table_name
ORDER BY id DESC
LIMIT 1;

or in Microsoft SQL Server:

SELECT TOP 1 id
FROM table_name
ORDER BY id DESC

40. How to select random rows from a table?

Using the RAND() function in combination with ORDER BY and LIMIT. In some SQL flavors, such as PostgreSQL, it's called RANDOM(). For example, the following code will return five random rows from a table in MySQL:

SELECT * FROM table_name
ORDER BY RAND()
LIMIT 5;