sqlunion
SQL Union is a fundamental concept in structured query language (SQL) that allows you to combine the results of multiple SELECT statements into a single result set. This can be particularly useful when you want to combine data from different tables or queries to get a complete picture of the data.
The UNION operator is used to combine the result sets of two or more SELECT statements. Each SELECT statement within the UNION must have the same number of columns and the columns must have compatible data types. The UNION operator removes duplicate rows from the combined result set.
For example
let's say we have two tables: employees and contractors. The employees table contains information about full-time employees
while the contractors table contains information about freelance contractors. We can use the UNION operator to combine data from both tables to get a complete list of all workers.
Here's an example query that combines data from the employees and contractors tables using the UNION operator:
```
SELECT emp_id
emp_name
emp_role
FROM employees
UNION
SELECT cont_id
cont_name
cont_role
FROM contractors;
```
In this query
we are selecting the employee ID
name
and role from the employees table and the contractor ID
name
and role from the contractors table. The UNION operator combines the results of these two SELECT statements into a single result set.
One important thing to note is that the columns selected in each SELECT statement within the UNION must be in the same order and have compatible data types. If the columns selected have different data types
the query will return an error.
Additionally
the UNION operator automatically removes duplicate rows from the combined result set. If you want to include duplicate rows in the result set
you can use the UNION ALL operator instead of just UNION.
Here's an example query that uses the UNION ALL operator to combine data from the employees and contractors tables
including duplicate rows:
```
SELECT emp_id
emp_name
emp_role
FROM employees
UNION ALL
SELECT cont_id
cont_name
cont_role
FROM contractors;
```
In this query
the UNION ALL operator combines the results of the two SELECT statements
including duplicate rows. This can be useful in cases where you want to include all records from both tables
even if there are duplicates.
Overall
the UNION operator in SQL is a powerful tool for combining data from multiple tables or queries into a single result set. It can help you create comprehensive reports
conduct complex analyses
and gain insights from your data. By mastering the UNION operator
you can enhance your SQL skills and become more proficient in querying databases.