SQL Server Union and SQL UNION all use methods _mssql

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 chooses a different value, that is, the union is gone heavy . If duplicate values are allowed, use UNIONall.

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.

The purpose of the UNION directive is to combine the results of two SQL statements. From this point of view, the UNION is somewhat similar to the JOIN, since these two instructions can be retrieved from multiple tables. The union simply joins the two results together to show that it is not connected to two tables ... The UNION 's syntax is as follows:

[SQL statement 1]
UNION
[SQL Statement 2] Let's say we have the following two tables,

store_information Table 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 table 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 All and union . Also assume that we have the following two forms,

store_information Table 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 table 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

eg

Copy Code code as follows:

SELECT Id,name,docpinyin from Doctor
UNION
SELECT 0 as ID, ' No doctor name ' as name, ' as Docpinyin

The result set is

SELECT Id,name,docpinyin from Doctor

The result set is

So: Union sets two results together