Shulou(Shulou.com)06/01 Report--

[example]

First, create two tables An and B. in order to facilitate understanding, both tables create only one field column, namely a_col and b_col, and add Table A.

Data 1, 4, 5, 9, add table B data 2, 3, 4, 5.

Enter the following sentence: select * from A union select * from B

Results: 1,4,5,9,2,3. (duplicate lines removed)

Enter the following statement: select * from A union all select * from B

Results: 1,4,5,9,2,3,4,5. (duplicate lines are not removed)

[conclusion]

Union: joins two result sets, excluding duplicate rows, and sorts the default rules

Union All: joins two result sets, including duplicate rows, without sorting

[notes]

Using union and union all must ensure that the result of each select collection has the same number of columns, and that the type of each column is

The same, but the column names do not need to be the same.

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