SQL Basics
Querying Data
Filtering Data
Joining Tables
SQL Functions
Modifying Data
Defining Data
SQL UNION
In this tutorial, you will learn how to combine two or more result sets from multiple queries using SQL UNION.
Once you know how to fetch your data using SELECT and FROM and the next step is to learn how to filter your data using WHERE clause.
In the previous tutorial, you learned how to retrieve all your rows from tables, but in real-world scenarios, we usually select only the rows which fulfill certain conditions like customers who come from a certain country.
Syntax
The basic syntax of the WHERE clause to filter the data returned by a query can be given with:
SELECT column_names
FROM table_name1
UNION [ALL]
SELECT column_names
FROM table_name2
It’s real easy to read in a plain-English way: select some columns from a table, except that the results will only include rows where that fulfill my conditions.
- The clauses need to be in this order e SELECT FROM WHERE
- The WHERE clause appears immediately after the FROM clause
- In WHERE clause, you can specify one or more comparisons and logical operators.
Examples
To understand the ORDER BY statement in a better way, let’s look at the following customers and orders tables in our tutorial database:
Now, let’s check out some examples that demonstrate how it actually works.
Filter Rows with WHERE Clause
We have the following task be to solve using SQL statements
The following SQL statement will returns all customers from customers table and order the result by the score column in ascending order.
You can have the same the result set by skipping the ASC, because it is the default option in ORDER BY.
SELECT
first_name,
last_name,
country
FROM customers
UNION ALL
SELECT
first_name,
last_name,
emp_country
FROM employees
After executing the above query, you’ll get the result set something like this:
SELECT
first_name,
last_name,
country
FROM customers
UNION
SELECT
first_name,
last_name,
emp_country
FROM employees
As you can see the output contains everything the whole customers tables including all rows and columns.
Similarly, you can use the DESC option to perform a sorting in descending order. The following statement will orders the result set by the numeric salary column in descending order.
SELECT
*
FROM customers
WHERE score > 500
Once you specify multiple columns after ORDER BY, the Database will sort the result by the first column, then the new ordered list will be sorted again by next column.
Operators in WHERE Clause
You can filter your results in a number of ways using comparison and logical operators, which you’ll learn about in the next tutorials. I summarized in the following table the most important ones.
