MYSQL-UNION

 grammar:

UNION operator used to combine  SELECT the results of two statements

SELECT statement
UNION [DISTINCT | ALL]
SELECT statement

UNIONSELECT The number of columns and the order of columns must be the same in the statements  in 

UNION is a binocular operator that requires two SELECT statements as operands. UNION can be followed by DISTINCT or ALL

If DISTINCT or ALL is not written, the default is UNION DISTINCT

UNION DISTINCT Or  UNION will filter out duplicate records in the result set

UNION ALL returns all results

Create test data

CREATE TABLE a (v INT);
CREATE TABLE b (v INT);
CREATE TABLE c (v INT);

INSERT INTO a VALUES (1), (2), (NULL), (NULL);
INSERT INTO b VALUES (2), (2), (NULL);
INSERT INTO c VALUES (3), (2);

a table: 

table b:

c-table:

example: 

SELECT * FROM a UNION SELECT * FROM b;

We can see that repeated NULLs have been deduplicated 

You can also use multiple UNIONs in conjunction

SELECT * FROM a UNION SELECT * FROM b UNION SELECT * FROM c;

In principle, we only require that the column names on both sides of the UNION be the same, not the same, but the official document requires that the column names must be the same, otherwise it may cause schema errors

However, if in general, when we UNION two different column names, the column name of the merged result is the same as the first operand of UNION