MySQL的'UNION'如何删除多列重复项？

Question

I know the main difference between UNION and UNION ALL is the former removes duplicated records, while the latter keeps all. My question is on how UNION removes duplicated record if there are multiple columns? For example, to union T1(A, B, C) and T2(A, B, C) ,

1. will MySQL consider (a1, b1, c1) and (a1, b2, c2) duplicates?
2. if there were duplicates in T1 itself, will MySQL remove this kind of duplicates?

Answer 1

During a UNION (not a UNION ALL ) query, MySQL first combines the records from all the tables involved, and then removes all duplicates.

Consider taking the UNION of the following two tables:

table1
col1 | col2
1    | 1
1    | 1
2    | 2
2    | 2
3    | 4

table2
col1 | col2
1    | 1
1    | 1
3    | 4

Consider the following query

SELECT * FROM table1
UNION
SELECT * FROM table2

The output would be this:

col1 | col2
1    | 1
2    | 2
3    | 4

In other words, simply because (1, 1) was duplicated within each separate table does not mean that it appears in duplicate in the result set. This output is only consistent with MySQL first combining all records and then removing duplicates. This has the side effect that if a record appear in duplicate within one of the tables, it also gets removed.

Demo here:

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