For this guide we’ll discuss the UNION Operator section of the SQL statement.
The UNION Operator is used to combine the results of multiple select statements into one result set.
The SQL statements must have the same number of columns in their Select Statement.
Basic Example
SQL Statement
SELECT 'aaaaa'
UNION
SELECT 'bbbbbbbbb';
Output
+-----------+
| aaaaa |
+-----------+
| aaaaa |
| bbbbbbbbb |
+-----------+
2 rows in set (0.00 sec)
Example using the student tables
SQL Statement
SELECT StudentID, FullName FROM student WHERE studentID BETWEEN 1 AND 5
UNION
SELECT studentID, studentEmailAddr FROM `student-contact-info` WHERE studentID BETWEEN 7 AND 8;
Output
+-----------+--------------------------------+
| StudentID | FullName |
+-----------+--------------------------------+
| 1 | Monique Davis |
| 2 | Teri Gutierrez |
| 3 | Spencer Pautier |
| 4 | Louis Ramsey |
| 5 | Alvin Greene |
| 7 | Maximo.Smith@freeCodeCamp.org |
| 8 | Michael.Roach@freeCodeCamp.ort |
+-----------+--------------------------------+
7 rows in set (0.00 sec)
SQL UNION ALL Operator
The UNION ALL operator is an extension to UNION operator where it should result you a A+B of rows in the ouptput assuming A and B is your input, in simple terms UNION ALL doesn’t deduplicate.
Basic Syntax
SQL Statement
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions]
UNION ALL
SELECT expression1, expression2, ... expression_n
FROM tables
[WHERE conditions];
As with all of these SQL things there is MUCH MORE to them than what’s in this introductory guide.
I hope this at least gives you enough to get started.
Please see the manual for your database manager and have fun trying different options yourself.