"Go" MySQL Federated Query Union and UNION ALL usage Introduction

MySQL's Federated query Command Union and UNION all, summarizing the use of syntax and considerations, as well as learning examples and project examples, require friends to refer to the roles and syntax of the next, Union, and Union all in conjunction with the Union ...

MySQL's Federated query Command Union and UNION all, summarizing the use of syntax and considerations, as well as learning examples and project examples, require friends to refer to the following
the function and syntax of Union and UNION ALL
The UNION is used to combine the result set of two or more SELECT statements and eliminate any duplicate rows in the table.
The SELECT statement inside the UNION must have the same number of columns, and the column must have a similar data type.
Also, the order of the columns in each SELECT statement must be the same.
SQL UNION Syntax:
The SQL script code is as follows:
SELECT column_name from table1
UNION
SELECT column_name from Table2
Note: By default, the UNION operator chooses a different value. If duplicate values are allowed, use UNION all.
When all is used with the Union (that is, union ALL), duplicate rows are not eliminated
SQL UNION All syntax
The SQL script code is as follows:
SELECT column_name from table1
UNION All
SELECT column_name from Table2
Note: In addition, the column name in the union result set is always equal to the column name in the first SELECT statement in the Union.
Note: 1, the column name in the UNION result set is always equal to the column name in the first SELECT statement
2. The SELECT statement inside 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
Ii. use of union and matters needing attention
Union: The union means to merge two or more query results together.
Requirements: Two queries must have the same number of columns
Recommendation: The type of column can be different, but the recommended query for each column, want the corresponding type to be the same
Data that can come from multiple tables: The column names taken out by multiple SQL statements can be inconsistent, with the column name of the first SQL statement as a standard.
If the rows taken in different statements are identical (in this case, the values for each column are the same), then union merges the same rows and eventually retains only one row. It is also possible to understand that union removes duplicate rows.
If you do not want to remove duplicate rows, you can use union ALL.
If there is an order by,limit in the clause, enclose it in parentheses (). After all clauses are recommended, the result of the final merge is sorted or filtered.
Such as:
The SQL script code is as follows:
(SELECT * from a ORDER by ID) union (SELECT * from B order ID);
In clauses, the order by needs to be used with limit to make sense. If not used with limit, it will be removed by the parser optimization analysis.
iii. Examples of learning
The original table is used in the following example:
Employees_china:
The SQL script code is as follows:
e_id E_name
Hua Zhang,
Wei Wang,
Carter, Thomas.
Yang, Ming
Employees_usa:
The SQL script code is as follows:
e_id E_name
Adams, John.
The Bush, George.
Carter, Thomas.
Gates, Bill.
Using the UNION command instance
List all the different employee names in China and the United States:
The SQL script code is as follows:
SELECT E_name from Employees_china
UNION
SELECT E_name from Employees_usa
Results:
The SQL script code is as follows:
E_name
Zhang, Hua
Wang, Wei
Carter, Thomas.
Yang, Ming
Adams, John.
Bush, George.
Gates, Bill.
Note: This command cannot list all employees in China and the United States. In the above example, we have two employees of the same name, and only one of them is listed. The UNION command will only pick a different value.
Using the UNION all command instance
The union ALL command is almost equivalent to the Union command, but the union ALL command lists all values.
The SQL script code is as follows:
SQL Statement 1
UNION All
SQL Statement 2
Instance:
List all employees in China and the United States:
The SQL script code is as follows:
SELECT E_name from Employees_china
UNION All
SELECT E_name from Employees_usa
Results
The SQL script code is as follows:
E_name
Zhang, Hua
Wang, Wei
Carter, Thomas.
Yang, Ming
Adams, John.
Bush, George.
Carter, Thomas.
Gates, Bill.

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"Go" MySQL Federated Query Union and UNION ALL usage Introduction