Union and union all operators in SQL

SQL UNION The operator

UNION The operator is used to merge two or more SELECT The result set of the statement .

Please note that ,UNION Inside SELECT Statements must have the same number of columns . Columns must also have similar data types . meanwhile , Every one of them SELECT The order of the columns in the statement must be the same .

SQL UNION grammar

SELECT column_name(s) FROM table_name1
UNION
SELECT column_name(s) FROM table_name2

SQL UNION ALL grammar

SELECT column_name(s) FROM table_name1
UNION ALL
SELECT column_name(s) FROM table_name2

in addition ,UNION The column name in the result set always equals UNION First of all SELECT Column names in statements .

Be careful ：

• UNION： Combine two result sets , Do not include repeating lines , At the same time, sort the default rules ;
• UNION All： Combine two result sets , Include repeating lines , No sorting .

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