sqlunion

SQLｕｎｉｏｎis 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.

Theｕｎｉｏｎoperator is used to combine the result sets of two or more ｓｅｌｅｃｔ statements. Each ｓｅｌｅｃｔ statement within theｕｎｉｏｎmust have the same number of columns and the columns must have compatible data types. Theｕｎｉｏｎoperator 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 theｕｎｉｏｎoperator 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 theｕｎｉｏｎoperator:

```

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. Theｕｎｉｏｎoperator 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 theｕｎｉｏｎmust 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

theｕｎｉｏｎoperator automatically removes duplicate rows from the combined result set. If you want to include duplicate rows in the result set

you can use theｕｎｉｏｎALL operator instead of just UNION.

Here's an example query that uses theｕｎｉｏｎALL 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

theｕｎｉｏｎALL 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

theｕｎｉｏｎoperator 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 ***yses

and gain insights from your data. By mastering theｕｎｉｏｎoperator

you can enhance your SQL skills and become more proficient in querying databases.