Set operator of gbase8s database in combined query

Set operators UNION、UNION ALL、INTERSECT and MINUS Operably specify Projection Phase in Clause

The result set of two queries with the same number of columns , And there are compatible data types in the corresponding columns of the two queries .

（MINUS Set operators have EXCEPT As its keyword synonym .MINUS and EXCEPT Operators from the same

Operands usually return the same result .）

These operators perform basic set operations on the result sets of two queries 、 Intersection and subtraction , These two result sets are these sets

The left and right operands of the operator ：

UNION The set operator combines qualified rows from two queries into a single result set , The result set consists of one

Non repeating rows returned by two queries or both queries .（ If you also include ALL keyword , be

UNION ALL The result set can include duplicate rows .）

INTERSECT Set operator compares result sets from two queries , But only return the result set of two queries at the same time

Non repeating lines in .

MINUS Set operator compares result sets from two queries , But only return the result set of the query on the left, but not

The non repeating rows in the result set of the query on the right .

In the context of business analysis , Set operators are useful . They can also be used in SELECT In the sentence , After the execution of such as

UPDATE、INSERT、DELETE or MERGE In this way DML After the operation , To check the integrity of the database . When

When you transfer data to the history table , Set operators can be used similarly , for example , When you need to make sure before deleting rows from the original table

When you recognize that there are correct data in the historical table .

All set operators have the same order of precedence . In complex queries that include multiple set operators , The order of precedence of the operators is

From left to right . Use parentheses to group set operators and their operands , If you need to override the set operator's default from

Left to right priority .

only UNION Set operators support ALL keyword . with INTERSECT、MINUS or EXCEPT Set operators

Of ALL Keyword is invalid , Only non repeating rows are returned from it .

When comparing rows to calculate the union or difference of a set , stay INTERSECT and MINUS Two of the operations NULL Value is considered to be

equal .