如何合并多个查询为一个查询?

select * from table_a where field_a = 1 LIMIT 0, 3;
select * from table_a where field_a = 2 LIMIT 0, 3;
select * from table_a where field_a = 3 LIMIT 0, 3;

如何合并以上３个SQL查询为１个查询？

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sqlselect * from table_a where field_a = 1 LIMIT 0, 3
union
select * from table_a where field_a = 2 LIMIT 0, 3
union
select * from table_a where field_a = 3 LIMIT 0, 3;

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SELECT * FROM table_a WHERE field_a IN (1, 2, 3)