A concise tutorial of SQL statements for Linux---UNION

The purpose of the UNION directive is to combine the results of two SQL statements. From this point of view, UNION is somewhat similar to join, because these two instructions can be retrieved from multiple tables. One limitation of the UNION is that the columns generated by the two SQL statements need to be of the same type of data. Also, when we use the UNION command, we will only see different data values (like SELECT DISTINCT).

The syntax of UNION is as follows:

[SQL statement 1]
UNION
[SQL Statement 2];

Let's say we have the following two tables,

store_information Form

Store_name	Sales	Txn_date
Los Angeles	1500	05-jan-1999
San Diego	250	07-jan-1999
Los Angeles	300	08-jan-1999
Boston	700	08-jan-1999

Internet Sales table

Txn_date	Sales
07-jan-1999	250
10-jan-1999	535
11-jan-1999	320
12-jan-1999	750

And we want to find out all the days that have turnover (sales). To achieve this, we use the following SQL statement:

SELECT txn_date from Store_information
UNION
SELECT txn_date from Internet_sales;

Results:

Txn_date

jan-05-1999

jan-07-1999

jan-08-1999

jan-10-1999

jan-11-1999

jan-12-1999

One thing to note is that if we use SELECT DISTINCT txn_date in any SQL statement (or both), we will get exactly the same result.

Linux measured results:

1. Create a table

2. Inserting table records

3.UNION (+) Two tables

A concise tutorial of SQL statements for Linux---UNION