[英]Create UNIQUE view from multiple tables where some fields are ignored
我想創建一個像這樣的視圖:
CREATE OR REPLACE VIEW all_cities AS
SELECT city,date_added FROM tableA
UNION DISTINCT
SELECT city,date_added FROM tableB
UNION DISTINCT
SELECT city,date_added FROM tableC;
all_cities表應該只包含其他表中的唯一城市條目,但是其他表可以在相同的城市中添加不同的日期,這會破壞我的聯盟,並導致在all_cities中具有所有這些重復項。 我仍然希望至少從合並結果之一中看到date_added字段(與哪一個無關)。
換句話說:如何創建一個視圖,該視圖從3個表中創建了一個唯一列表,但是在進行惟一時忽略了date_added字段。
謝謝
最好的選擇方式
select city, min(date_added)
from (
SELECT city,date_added FROM tableA
UNION DISTINCT
SELECT city,date_added FROM tableB
UNION DISTINCT
SELECT city,date_added FROM tableC
)A
GROUP BY city
但是MySQL不允許在view語句中使用子查詢,但是您可以在view語句中使用view,因此我將為該解決方案提供2個視圖。
CREATE OR REPLACE VIEW all_cities_support_view AS
SELECT city,date_added FROM tableA
UNION DISTINCT
SELECT city,date_added FROM tableB
UNION DISTINCT
SELECT city,date_added FROM tableC;
CREATE OR REPLACE VIEW all_cities AS
SELECT city, min(date_added) as date_added
FROM all_cities_support_view
GROUP BY city;
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