Data from two tables into one view

问题

Is it possible to grab data from two tables (that have the same fields) into one view. Basically, so the view sees the data as if it was one table.

回答1:

Yes, using a UNION -

CREATE VIEW vw_combined AS
   SELECT * FROM TABLE1
   UNION ALL
   SELECT * FROM TABLE2

...requires that there be the same number of columns, and the data types match at each position.

..preferrably, using a JOIN:

CREATE VIEW vw_combined AS
   SELECT * 
    FROM TABLE1 t1
    JOIN TABLE2 t2 ON t2.col = t1.col

But I want to warn against depending on views - if not materialized, they are only prepared SQL statements. There's no performance benefit, and can negatively impact performance if you build a view based on another. Also, views are brittle - they can change, and you won't know until using a supporting view if there are issues.

回答2:

create or replace view view_name as
select * from table_1
union all select * from table_2

Note: The columns in the view are set at the time the view is created. Adding columns to table_1 and table_2 after view creation will not show up in view_name. You will need to re-run the above DDL to get new columns to show up.

If you want duplicate rows to be collasped to single rows (but potentially more work for the server):

create or replace view view_name as
select * from table_1
union select * from table_2

Generally it is bad form to use * in the select list, but assuming that the queries using the view are going to choose just what they need, I would use it here instead of explicitily naming all the columns. (Especially since I wouldn't want to have to add the column names when table_1 and table_2 change.)

来源：https://stackoverflow.com/questions/3267360/data-from-two-tables-into-one-view

标签

sql

Oracle

sql-view