Structured Query Language (SQL)

Structured Query Language (SQL) is a domain-specific language used to manage data, especially in a relational database management system (RDBMS). It is particularly useful in handling structured data, i.e., data incorporating relations among entities and variables.
— Wikipedia

NOTE

Follow-up information mostly added from Ben Forta’s SQL book ¹ and quick-SQL-cheatsheet ². In examples, I use sqlite databases from w3school DB and Ben Forta’s book.

SQL is a language of structured queries, created to interaction with databases.

SQL is almost universal language, mostly all databases support it (with some dialect specific differences for non ANSI SQL features).

SQL is relatively simple but powerful language, it’s consist of limited set of English keywords. It’s possible to make very complex operations on databases with it.

Never use SQL reserved keywords as table or column name.

Query language used to manage data in a relational RDBMS (database).

CRUD term is short name of Create, Read, Update, Delete operations (queries) of data, based on specific criteria.

High-level overview of SQL language:

SQL overview.excalidraw

SQL components diagram

DDL, DQL, DML, DCL and TCL Commands

DDL is short name of Data Definition Language, which deals with database schemas and descriptions, of how the data should reside in the database. DDL example:

CREATE TABLE users (
    search_id INT NULL,
    about_profile TEXT NULL,
    courses TEXT NULL,
);

DQL is short name of Data Query Language which used for performing queries on the data within schema objects. The purpose of the DQL Command is to get some schema relation based on the query passed to it. SELECT statement is used to retrieve data from the database.

DML is short name of Data Manipulation Language which deals with data manipulation, and includes most common SQL statements such INSERT, UPDATE, DELETE etc., and it is used to store, modify, delete and update data in database. Example:

INSERT INTO employees (first_name, last_name, fname) VALUES (
    'John', 'Capita', 'xcapit00'
);

DCL is short name of Data Control Language which includes commands such as GRANT, and mostly concerned with rights, permissions and other controls of the database system. DCL example:

REVOKE INSERT ON *.* FROM 'jeffrey'@'localhost';
REVOKE 'role1', 'role2' FROM 'user1'@'localhost', 'user2'@'localhost';
REVOKE SELECT ON world.* FROM 'role3';
GRANT ALL ON db1.* TO 'jeffrey'@'localhost';
GRANT 'role1', 'role2' TO 'user1'@'localhost', 'user2'@'localhost';
GRANT SELECT ON world.* TO 'role3';

The Transaction Control Language (TCL) is used in conjunction with the Data Manipulation Language to control the processing and exposure of changes, in other words control the transactions.

How to create the people table with the following fields (sqlite):

id - Integer
handle - Text
name - Text
age - Integer
balance - Integer
is_admin - boolean

create table people (
  id INTEGER,
  handle TEXT,
  name TEXT,
  age INTEGER,
  balance INTEGER,
  is_admin BOOLEAN
);

The SQL SELECT Statement

SELECT is used to extract data from one or multiple tables, usually with specific conditions.

Select CustomerName, CategoryName from customers and categories (no extra filtering and join).

SELECT CustomerName, CategoryName FROM customers, categories;

CustomerName                          CategoryName
------------------------------------  --------------
Alfreds Futterkiste                   Beverages
Alfreds Futterkiste                   Condiments
Alfreds Futterkiste                   Confections

Select unique CustomerName from customers (filters away duplicate values and returns rows of specified column).

SELECT DISTINCT CustomerName FROM customers;

CustomerName
------------------------------------
Alfreds Futterkiste
Ana Trujillo Emparedados y helados
Antonio Moreno Taquería

For operators that occur at the same precedence level within an expression, evaluation proceeds left to right, with the exception that assignments evaluate right to left.

Select CustomerID, CustomerName from users where CustomerId != 3 and 1 < id < 10
AND has precedence over OR

SELECT
    CustomerID,
    CustomerName
FROM `customers`
WHERE CustomerId != 3 AND CustomerId > 1 AND CustomerId < 10;

CustomerID  CustomerName
----------  ----------------------------------
2           Ana Trujillo Emparedados y helados
4           Around the Horn
5           Berglunds snabbköp

Lists the suppliers with a product price less than 20, use EXISTS clause.

ProductID	ProductName	SupplierID	CategoryID	Unit	Price
1	Chais	1	1	10 boxes x 20 bags	18

SupplierID	SupplierName	ContactName	Address	City	PostalCode	Country
1	Exotic Liquid	Charlotte Cooper	49 Gilbert St.	London	EC1 4SD	UK

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

SupplierName
---------------------------------
Exotic Liquid
New Orleans Cajun Delights
Tokyo Traders

The EXISTS operator is used to test for the existence of any record in a subquery. The EXISTS operator returns TRUE if the subquery returns one or more records.

ORDER BY: used to sort the result-set in ascending or descending order. Select OrderDetailID, OrderID, from order_details, sort by OrderID in ascending order and ProductID in descending order.

SELECT OrderDetailID, OrderID
FROM order_details
ORDER BY OrderID ASC, ProductID DESC;

OrderDetailID  OrderID
-------------  -------
3              10248
2              10248
1              10248

The LIMIT clause is used to place an upper bound on the number of rows returned by the entire SELECT statement. How to select the 10-20 rows from orders?
Some databases support SELECT TOP clause, but not all. I’ll use LIMIT clause instead.

SELECT * FROM orders LIMIT 10, 20;

OrderID  CustomerID  EmployeeID  OrderDate   ShipperID
-------  ----------  ----------  ----------  ---------
10258    20          1           1996-07-17  1
10259    13          4           1996-07-18  3
10260    55          4           1996-07-19  1

LIKE operator used in a WHERE clause to search for a specific pattern in a column. How to find all customers, where CustomerName contains an and their address start with “a” and are at least 3 characters in length, and address starting have a in second position.

SELECT * FROM customers
WHERE CustomerName LIKE '%an%' AND Address LIKE '_a%';

CustomerName             City
-----------------------  --------------
Antonio Moreno Taquería  México D.F.
Island Trading           Cowes
Lehmanns Marktstand      Frankfurt a.M.

% (percent sign) is a wildcard character that represents zero, one, or multiple characters. _ (underscore) is a wildcard character that represents a single character.

LIKE ‘a%’ (find any values that start with “a”)
LIKE ‘%a’ (find any values that end with “a”)
LIKE ‘%or%’ (find any values that have “or” in any position)
LIKE ‘_r%’ (find any values that have “r” in the second position)
LIKE ‘a_%_%’ (find any values that start with “a” and are at least 3 characters in length)
LIKE ‘[a-c]%’ (find any values starting with “a”, “b”, or “c”

IN operator allows you to specify multiple values in a WHERE clause essentially the IN operator is shorthand for multiple OR conditions. Search for all customers where their country is one of the following:

Germany
France
UK

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

SELECT column_names FROM table_name WHERE column_name IN (value1, value2, …);
SELECT column_names FROM table_name WHERE column_name IN (SELECT STATEMENT);

BETWEEN operator selects values within a given range inclusive (begin and end values are included). Selects all orders with an OrderDate between '1996-07-04' and '1996-07-31':

SELECT * FROM Orders
WHERE OrderDate BETWEEN '1996-07-04' AND '1996-07-31';

OrderID  CustomerID  EmployeeID  OrderDate   ShipperID
-------  ----------  ----------  ----------  ---------
10248    90          5           1996-07-04  3
10249    81          6           1996-07-05  1
10250    34          4           1996-07-08  2

SELECT * FROM Products WHERE (column_name BETWEEN value1 AND value2) AND NOT column_name2 IN (value3, value4);

If a field in a table is optional, it is possible to insert a new record or update a record without adding a value to this field. Then, the field will be saved with a NULL value. How to list all customers with a NULL value in the Address field:

SELECT *
FROM Customers
WHERE Address IS NULL;

no rows in result set

SELECT * FROM table_name WHERE column_name IS NULL;
SELECT * FROM table_name WHERE column_name IS NOT NULL;

AS aliases are used to assign a alias (temporary name) to a table or column. An alias only exists for the duration of the query. Create an alias named “Address” that combine four columns (Address, PostalCode, City and Country):

SELECT CustomerName, Address + ', ' + PostalCode + ' ' + City + ', ' + Country AS Address
FROM Customers;

To get the SQL statement above to work in MySQL use CONCAT()

SELECT column_name AS alias_name FROM table_name;
SELECT column_name FROM table_name AS alias_name;
SELECT column_name AS alias_name1, column_name2 AS alias_name2;
SELECT column_name1, column_name2 + ‘, ‘ + column_name3 AS alias_name;

UNION set operator used to combine the result-set of two or more SELECT statements. Each SELECT statement within UNION must have the same number of columns. The columns must have similar data types. The columns in each SELECT statement must also be in the same order. UNION operator only selects distinct values, UNION ALL will allow duplicates. Return the cities (duplicate values also) from both the "Customers" and the "Suppliers" table:

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

SELECT columns_names FROM table1 UNION SELECT column_name FROM table2;

INTERSECT set operator which is used to return the records that two SELECT statements have in common. Generally used the same way as UNION (both queries has same columns). Returns a list of cities that have both customers and suppliers.

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

SELECT columns_names FROM table1 INTERSECT SELECT column_name FROM table2;

EXCEPT set operator used to return all the records in the first SELECT statement that are not found in the second SELECT statement. Generally used the same way as UNION. Select CustomerID from customers without any orders:

SELECT CustomerID
FROM customers
EXCEPT
SELECT CustomerID
FROM orders;

SELECT columns_names FROM table1 EXCEPT SELECT column_name FROM table2;

The ANY and ALL operators allow you to perform a comparison between a single column value and a range of other values.

ANY means that the condition will be true if the operation is true for any of the values in the range. The ANY operator:

returns a boolean value as a result
returns TRUE if ANY of the subquery values meet the condition ANY means that the condition will be true if the operation is true for any of the values in the range.
List the ProductName if it finds ANY records in the OrderDetails table has Quantity equal to 10 (this will return TRUE because the Quantity column has some values of 10). SQLite does not support the ANY operator.

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

The ALL operator:

returns a boolean value as a result
returns TRUE if ALL of the subquery values meet the condition
is used with SELECT, WHERE and HAVING statements ALL means that the condition will be true only if the operation is true for all values in the range. Lists the ProductName if ALL the records in the OrderDetails table has Quantity equal to 10. This will of course return FALSE because the Quantity column has many different values (not only the value of 10). SQLite doesn’t support ALL in the WHERE clause.

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

Lists ALL the product names, including duplicates (using ALL).
ALL is the default, and most people write just SELECT instead of SELECT ALL.

SELECT ALL ProductName
FROM Products
WHERE TRUE;

The GROUP BY statement groups rows that have the same values into summary rows, like “find the number of customers in each country”. The GROUP BY statement is often used with aggregate functions (COUNT(), MAX(), MIN(), SUM(), AVG()) to group the result-set by one or more columns. List the number of orders (Orders.OrderID) sent by each shipper (Shippers.ShipperName):

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

HAVING this clause was added to SQL because the WHERE keyword could not be used with aggregate functions. How lists the number of customers in each country, sorted high to low (Only include countries with more than 5 customers)?

SELECT COUNT(CustomerID), Country
FROM Customers
GROUP BY Country
HAVING COUNT(CustomerID) > 5
ORDER BY COUNT(CustomerID) DESC;

WITH often used for retrieving hierarchical data or re-using temp result set several times in a query. Also referred to as "Common Table Expression" WITH RECURSIVE cte AS (
SELECT c0.* FROM categories AS c0 WHERE id = 1 # Starting point
UNION ALL
SELECT c1.* FROM categories AS c1 JOIN cte ON c1.parent_category_id = cte.id
)
SELECT *
FROM cte

Data Modification Queries

INSERT INTO used to insert new records/rows in a table How to insert multiple rows of data with INSERT?
We use the INSERT INTO statement, but with multiple values:

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

UPDATE used to modify the existing records in a table. How to update the ContactName to "Juan" for all customers records where country is "Mexico":

UPDATE Customers
SET ContactName='Juan' -- place here additional ", column = value"
WHERE Country='Mexico';

DELETE used to delete existing records/rows in a table Recommended using it with WHERE clause. How to delete the customer "Alfreds Futterkiste" from the "Customers" table:

DELETE FROM Customers WHERE CustomerName='Alfreds Futterkiste';

Reporting Queries

COUNT returns the number of occurrences. Use the COUNT() function and the GROUP BY clause, to return the number of records for each category in the Products table:

SELECT COUNT(*) AS [Number of records], CategoryID
FROM Products
GROUP BY CategoryID;

MIN() and MAX() returns the smallest/largest value of the selected column. Use the MIN() function and the GROUP BY clause, to return the smallest price for each category in the Products table:

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

AVG() returns the average value of a numeric column. List all records with a higher price than average, we can use the AVG() function in a sub query:

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

SUM() returns the total sum of a numeric column. Use the SUM() function and the GROUP BY clause, to return the Quantity for each OrderID in the OrderDetails table:

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

Join Queries

SQL Join Queries

The joined table will contain all records from both the tables and fill in NULL values for missing matches on either side.

**INNER JOIN returns records that have matching value in both tables. Create the following SQL statement (that contains a INNER JOIN), that selects records that have matching values in orders and customers tables, CustomerID is less than 10:

-- Orders                                -- Customers
SELECT CustomerID, OrderID, OrderDate    SELECT CustomerID, CustomerName
FROM orders                              FROM Customers
WHERE CustomerID < 10;                   WHERE CustomerID < 10;
 
CustomerID  OrderID  OrderDate           CustomerID  CustomerName
----------  -------  ----------          ----------  ----------------------------------
2           10308    1996-09-18          2           Ana Trujillo Emparedados y helados
3           10365    1996-11-27          3           Antonio Moreno Taquería
4           10355    1996-11-15          4           Around the Horn
4           10383    1996-12-16          5           Berglunds snabbköp
5           10278    1996-08-12          6           Blauer See Delikatessen
5           10280    1996-08-14          7           Blondel père et fils
5           10384    1996-12-16          8           Bólido Comidas preparadas
7           10265    1996-07-25          9           Bon app
7           10297    1996-09-04
7           10360    1996-11-22
7           10436    1997-02-05
8           10326    1996-10-10
9           10331    1996-10-16
9           10340    1996-10-29
9           10362    1996-11-25
 
-- Customers and Orders
SELECT Orders.OrderID, Orders.OrderDate, Orders.CustomerID, Customers.CustomerName
FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID
WHERE Orders.CustomerID < 10;
 
OrderID  OrderDate   CustomerID  CustomerName
-------  ----------  ----------  ----------------------------------
10308    1996-09-18  2           Ana Trujillo Emparedados y helados
10365    1996-11-27  3           Antonio Moreno Taquería
10355    1996-11-15  4           Around the Horn
10383    1996-12-16  4           Around the Horn
10278    1996-08-12  5           Berglunds snabbköp
10280    1996-08-14  5           Berglunds snabbköp
10384    1996-12-16  5           Berglunds snabbköp
10265    1996-07-25  7           Blondel père et fils
10297    1996-09-04  7           Blondel père et fils
10360    1996-11-22  7           Blondel père et fils
10436    1997-02-05  7           Blondel père et fils
10326    1996-10-10  8           Bólido Comidas preparadas
10331    1996-10-16  9           Bon app
10340    1996-10-29  9           Bon app
10362    1996-11-25  9           Bon app

SELECT column_names FROM table1 INNER JOIN table2 ON table1.column_name=table2.column_name;
SELECT table1.column_name1, table2.column_name2, table3.column_name3 FROM ((table1 INNER JOIN table2 ON relationship) INNER JOIN table3 ON relationship);

LEFT (OUTER) JOIN returns all records from the left table (table1), and the matched records from the right table (table2). Select all customers (CustomerName), and any orders (OrderID) they might have:

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

RIGHT (OUTER) JOIN returns all records from the right table (table2), and the matched records from the left table (table1). The result is 0 records from the left side, if there is no match. Return all employees (LastName, FirstName), and any orders (OrderID) they might have placed:

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

FULL (OUTER) JOIN returns all records when there is a match in either left or right table Selects all customers (CustomerName), and all orders (OrderID):

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

Self JOIN a regular join, but the table is joined with itself, as if the table were two tables, temporarily renaming at least one table in the SQL statement. Match customers (CustomerName) that are from the same city (City):

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;

CARTESIAN JOIN: returns the Cartesian product of the sets of records from the two or more joined tables. It’s essentially a multiplication operation between the rows of the involved tables. Create a new result set that contains every possible combination of a customer (CustomerName) and order (OrderId).

SELECT Customers.CustomerName, Orders.OrderID
FROM Customers
CROSS JOIN Orders;

View Queries

What is a view? Creates a view that shows all customers from Brazil:
In SQL, a view is a virtual table based on the result-set of an SQL statement.

CREATE VIEW [Brazil Customers] AS
SELECT CustomerName, ContactName
FROM Customers
WHERE Country = 'Brazil';
 
SELECT * FROM [Brazil Customers];

Is it possible to overwrite a view?
Yes, by using CREATE OR REPLACE VIEW:

CREATE OR REPLACE VIEW [Brazil Customers] AS
SELECT CustomerName, ContactName, City
FROM Customers
WHERE Country = 'Brazil';

How to drop the “Brazil Customers” view:

DROP VIEW [Brazil Customers];

Altering Table Queries

How to add a "Email" column to the "Customers" table?

ALTER TABLE Customers
ADD Email varchar(255);

How to delete the "Email" column from the "Customers" table?

ALTER TABLE Customers
DROP COLUMN Email;

How to rename the "City" column to "Location"?

ALTER TABLE Customers
RENAME COLUMN City TO Location;

How to change the data type of the "City" column to "varchar(100)" in Customers table?

# MySQL dialect
ALTER TABLE Customers
MODIFY COLUMN City varchar(100);

Creating Table Query

Create a table called "Persons" that contains five columns: PersonID, LastName, FirstName, Address, and City:

CREATE TABLE Persons (
    PersonID int,
    LastName varchar(255),
    FirstName varchar(255),
    Address varchar(255),
    City varchar(255)
);

Security

Methods to avoid SQL injection?

First refer to language/library documentation, how to use placeholder for user’s input. Avoiding SQL injection risk - The Go Programming Language
Filter query elements from user’s input (or need just avoid this) with elements allowlist.

Inom Turdikulov

Structured Query Language (SQL)

DDL, DQL, DML, DCL and TCL Commands

The SQL SELECT Statement

Data Modification Queries

Reporting Queries

Join Queries

View Queries

Altering Table Queries

Creating Table Query

Security

External links

Graph View

Backlinks

Table of Contents

Inom Turdikulov

Structured Query Language (SQL)

DDL, DQL, DML, DCL and TCL Commands

The SQL SELECT Statement

Data Modification Queries

Reporting Queries

Join Queries

View Queries

Altering Table Queries

Creating Table Query

Security

External links

Footnotes

Graph View

Backlinks

Table of Contents