SQL Tutorial 18 Union Clause | Ties Clause

SQL Tutorial 18 

What is SQL Union Clause with  Example?

 The SQL UNION clause is used to combine the results of two or more SELECT statements without including duplicate rows. It is important to note that the number and order of columns in all SELECT statements must be the same. The syntax for the UNION clause is as follows:

SELECT column1, column2, …

FROM table1

WHERE condition

UNION

SELECT column1, column2, …

FROM table2

WHERE condition;

Here’s a simple example to illustrate the usage of the UNION clause. Consider two tables, employees and customers, each with columns id and name. We want to retrieve a list of unique names from both tables:

— Creating sample tables

CREATE TABLE employees (

    id INT,

    name VARCHAR(255)

);

CREATE TABLE customers (

    id INT,

    name VARCHAR(255)

);

— Inserting sample data

INSERT INTO employees (id, name) VALUES (1, ‘John’);

INSERT INTO employees (id, name) VALUES (2, ‘Jane’);

INSERT INTO employees (id, name) VALUES (3, ‘Bob’);

INSERT INTO customers (id, name) VALUES (4, ‘Alice’);

INSERT INTO customers (id, name) VALUES (5, ‘John’);

INSERT INTO customers (id, name) VALUES (6, ‘Jane’);

— Using UNION to combine results and retrieve unique names

SELECT name FROM employees

UNION

SELECT name FROM customers;

In this example, the UNION operator is used to combine the results of the two SELECT statements. The final result set contains unique names from both the employees and customers tables.

If you want to include all rows, including duplicates, you can use the UNION ALL clause instead of UNION. The UNION ALL clause is generally faster than UNION because it doesn’t perform the additional step of removing duplicates. However, if you want to eliminate duplicate rows, you should use UNION.

 SQL ALL and Any with example

 In SQL, the ALL and ANY operators are used in conjunction with comparison operators to perform comparisons against a set of values. Let’s explore each of them with examples:

SQL ALL Operator:

The ALL operator is used to compare a value to all values returned by a subquery. The result is TRUE if the specified comparison holds true for all the values; otherwise, it returns FALSE.

Syntax:

expression operator ALL (subquery)

Example:

Suppose you have a table products with columns product_id and price, and you want to find products with a price greater than all the prices in a certain range. Here’s an example:

— Find products with a price greater than all prices in a certain range

SELECT product_id, product_name, price

FROM products

WHERE price > ALL (SELECT price FROM products WHERE category = ‘Electronics’);

In this example, the subquery retrieves all prices for products in the ‘Electronics’ category, and the main query selects products with a price greater than all those prices.

SQL ANY Operator:

The ANY operator is used to compare a value to any value in a set of values. The result is TRUE if the specified comparison holds true for at least one value; otherwise, it returns FALSE.

Syntax:

expression operator ANY (subquery)

Example:

Suppose you want to find products with a price greater than any price in a certain range. Here’s an example:

— Find products with a price greater than any price in a certain range

SELECT product_id, product_name, price

FROM products

WHERE price > ANY (SELECT price FROM products WHERE category = ‘Clothing’);

In this example, the subquery retrieves all prices for products in the ‘Clothing’ category, and the main query selects products with a price greater than any of those prices.

It’s worth noting that ANY and ALL can be used with various comparison operators such as =, >, <, >=, <=, etc., depending on the specific requirements of the query.

SQL Aliases with example

In SQL, aliases are used to give a table or column a temporary name, making the result set more readable or concise. Aliases can be particularly useful when dealing with complex queries, calculations, or when joining tables. Here are examples of using aliases in SQL:

Table Alias:

— Using a table alias to shorten table names

SELECT e.employee_id, e.employee_name, d.department_name

FROM employees AS e

JOIN departments AS d ON e.department_id = d.department_id;

In this example, e and d are aliases for the employees and departments tables, respectively. Using aliases can make the SQL statement more concise and easier to read.

Column Alias:

— Using column aliases to rename result set columns

SELECT product_id AS ID, product_name AS Name, unit_price AS Price

FROM products;

Here, AS is used to assign aliases to the columns in the result set. This can be helpful when you want to present the data with more user-friendly or descriptive names.

Alias in Calculations:

— Using aliases in calculations

SELECT order_id, quantity, unit_price, quantity * unit_price AS total_price

FROM order_details;

In this example, an alias (total_price) is used for the result of a calculation (quantity * unit_price). This makes the result set more readable, and the alias can be referenced in the query.

Alias in GROUP BY:

— Using aliases in GROUP BY

SELECT department_id, AVG(salary) AS average_salary

FROM employees

GROUP BY department_id;

Here, AVG(salary) is given the alias average_salary for better readability. The alias can be used in the SELECT clause or in other parts of the query.

Alias in WHERE Clause:

— Using aliases in the WHERE clause

SELECT product_id, product_name, unit_price * 0.9 AS discounted_price

FROM products

WHERE discounted_price < 50;

Although some database systems might not allow using aliases in the WHERE clause directly, you can use a subquery or a derived table to achieve a similar result.

Aliases make SQL statements more readable and provide a way to reference columns or calculations using user-friendly names. They are especially useful in large and complex queries.

SQL Wildcard with example

In SQL, wildcards are special characters used in conjunction with the LIKE operator to perform pattern matching in string comparisons. There are two common wildcard characters: % (percent sign) and _ (underscore).

% Wildcard:

The % wildcard represents zero or more characters.

Example:

— Selecting all employees whose names start with ‘J’

SELECT * FROM employees

WHERE employee_name LIKE ‘J%’;

This query retrieves all rows from the employees table where the employee_name starts with the letter ‘J’.

_ Wildcard:

The _ wildcard represents a single character.

Example:

— Selecting all employees whose names have ‘a’ as the second character

SELECT * FROM employees

WHERE employee_name LIKE ‘_a%’;

This query retrieves all rows from the employees table where the second character of the employee_name is ‘a’.

Using both % and _:

You can combine % and _ for more complex pattern matching.

Example:

— Selecting all employees whose names have ‘a’ as the second character and end with ‘son’

SELECT * FROM employees

WHERE employee_name LIKE ‘_a%son’;

This query retrieves all rows from the employees table where the employee_name has ‘a’ as the second character and ends with ‘son’.

Wildcards with NOT LIKE:

You can also use the NOT LIKE operator to find rows that do not match a specified pattern.

Example:

— Selecting all employees whose names do not start with ‘M’

SELECT * FROM employees

WHERE employee_name NOT LIKE ‘M%’;

This query retrieves all rows from the employees table where the employee_name does not start with the letter ‘M’.

Wildcards are powerful tools for searching and filtering data based on patterns within string columns. Keep in mind that the specific syntax might vary slightly depending on the database system you are using (e.g., MySQL, PostgreSQL, SQL Server).

SQL Ties Clause with example

As of my last knowledge update in January 2022, there is no specific “Ties Clause” in standard SQL. However, when dealing with queries that involve ranking functions like RANK(), DENSE_RANK(), or ROW_NUMBER(), you might encounter situations where ties (equal values) need to be handled.

For example, consider the following query using the RANK() window function:

SELECT

  employee_name,

  salary,

  RANK() OVER (ORDER BY salary DESC) AS salary_rank

FROM

  employees;

In this query, RANK() is used to assign a rank to each employee based on their salary in descending order. If two or more employees have the same salary, they will be assigned the same rank, and the next rank will be skipped.

If you want to handle ties differently, you might need to use additional criteria to break the tie. For instance, you could modify the query to consider the employee’s hire date in case of a tie:

SELECT

  employee_name,

  salary,

  hire_date,

  RANK() OVER (ORDER BY salary DESC, hire_date) AS salary_rank

FROM

  employees;

In this example, if two employees have the same salary, the one with the earlier hire date will get a lower rank.

It’s important to note that handling ties can depend on the specific requirements of your query and the database system you’re using. Different database systems may have variations in the syntax and behavior of ranking functions.

If you have a specific use case or a different context in mind regarding the “Ties Clause,” please provide more details, and I’ll do my best to assist you.

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