How to use and distinguish Union and union all in MySQL

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.

The syntax is as follows:

SELECTcolumn_name FROMtable1

UNION

SELECT  column_name  FROM  table2 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 same3. If the clause has an order by,limit, enclose it in parentheses (). After all clauses are recommended, the result of the final merge is sorted or filtered. Example:Table A data: E_ID E_Name 01 Zhang, Hua 02 Wang, Wei 03 Carter, Thomas 04 Yang, Ming Employees_USA:B Table Data: E_ID E_Name 01 Adams, John 02 Bush, George 03 Carter, Thomas 04 Gates, BillQuery statement: SELECT  E_Name  FROM  Employees_China UNION SELECT  E_Name  FROM  Employees_USAResult set: E_Name Zhang, Hua Wang, Wei Carter, Thomas Yang, Ming Adams, John Bush, George Gates, Bill

Usage and differences between union and unions in MySQL