SQL Union and UNION ALL syntax and instances

SQL Union and UNION ALL syntax and instances
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SQL UNION operator
The union operator is used to merge the result sets of two or more SELECT statements.

Note that the SELECT statement within the Union must have the same number of columns. The column must also have a similar data type. Also, the order of the columns in each SELECT statement must be the same.

SQL Union syntax
Select COLUMN_NAME (s) from table_name1
Union
Select COLUMN_NAME (s) from table_name2
Note: By default, the union operator picks a different value. If duplicate values are allowed, use UNION ALL.

SQL UNION ALL syntax
Select COLUMN_NAME (s) from table_name1
UNION ALL
Select COLUMN_NAME (s) from table_name2
In addition, the column names in the union result set are always equal to the column names in the first SELECT statement in the Union.

SQL Union and UNION ALL application instances

The union simply joins the two results together to show that it is not linked to two tables.
The Union syntax is as follows: [SQL statement 1]
Union
[SQL Statement 2] Suppose we have the following two tables,
Store_information form Store_name Sales Date
Los Angeles $1500 jan-05-1999
San Diego $ jan-07-1999
Los Angeles $300 jan-08-1999
Boston $700 jan-08-1999
Internet Sales Form Date Sales
jan-07-1999 $
jan-10-1999 $535
jan-11-1999 $320
jan-12-1999 $750
And we're going to find all the sales days. To achieve this, we use the following SQL statement:
Select Date from Store_information
Union
Select date from internet_sales results:
Date
jan-05-1999
jan-07-1999
jan-08-1999
jan-10-1999
jan-11-1999
jan-12-1999
It is worth noting that if we use "SELECT DISTINCT date" in any of the SQL statements (or both), we will get exactly the same result.
SQL UNION ALL
UNION ALL the purpose of this instruction is to merge the results of two SQL statements together. The union all and the Union differ in that the Union all lists each qualifying data, regardless of whether the data value is duplicated or not. The syntax for union ALL is as follows: [SQL statement 1]
UNION ALL
[SQL Statement 2] We use the same example as the previous page to show the difference between Union and union. Also assume that we have the following two tables:
Store_information form Store_name Sales Date
Los Angeles $1500 jan-05-1999
San Diego $ jan-07-1999
Los Angeles $300 jan-08-1999
Boston $700 jan-08-1999
Internet Sales Form Date Sales
jan-07-1999 $
jan-10-1999 $535
jan-11-1999 $320
jan-12-1999 $750
And we need to find out where the store turnover and network turnover date. To achieve this, we use the following SQL statement:
Select Date from Store_information
UNION ALL
Select date from internet_sales results:
Date
jan-05-1999
jan-07-1999
jan-08-1999
jan-08-1999
jan-07-1999
jan-10-1999
jan-11-1999
jan-12-1999