One Limit of Union is that the columns generated by two SQL statements must be of the same data type. In addition, when we use the Union command, we only see different data values (similar to select distinct ). Union only Concatenates the two results and displays them together. It does not join two tables.
The syntax of union is as follows: [SQL statement 1]
Union
[SQL statement 2] Suppose we have the following two tables,
Store_information table store_name sales date
Los Angeles $1500 Jan-05-1999
San Diego $250 Jan-07-1999
Los Angeles $300 Jan-08-1999
Boston $700 Jan-08-1999
Internet sales table date sales
Jan-07-1999 $250
Jan-10-1999 $535
Jan-11-1999 $320
January 12-1999 $750
We need to find out all the days with a turnover. To achieve this goal, we use the following SQL statement:
Select date from store_information
Union
Select date from internet_sales result:
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 SQL statement (or both sentences), we will get the same result.
SQL Union all
The purpose of the Union all command is to combine the results of two SQL statements. The difference between Union all and union is that Union all lists each qualified data, regardless of whether the data value is repeated. The syntax of Union all is as follows: [SQL statement 1]
Union all
[SQL statement 2] we use the same example on the previous page to show the differences between Union all and union. Let's assume that we have two tables:
Store_information table store_name sales date
Los Angeles $1500 Jan-05-1999
San Diego $250 Jan-07-1999
Los Angeles $300 Jan-08-1999
Boston $700 Jan-08-1999
Internet sales table date sales
Jan-07-1999 $250
Jan-10-1999 $535
Jan-11-1999 $320
January 12-1999 $750
However, we need to find out the days when there are store turnover and network turnover. To achieve this goal, we use the following SQL statement:
Select date from store_information
Union all
Select date from internet_sales result:
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
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