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今天我的注意力不集中,或者Slick的入門文檔沒有很好的設計。 所以在查詢/工會節他們有這樣的片段:Slick文檔中的錯誤示例?
val q1 = coffees.filter(_.price < 8.0)
val q2 = coffees.filter(_.price > 9.0)
val unionQuery = q1 union q2
// compiles to SQL (simplified):
// select x8."COF_NAME", x8."SUP_ID", x8."PRICE", x8."SALES", x8."TOTAL"
// from "COFFEES" x8
// where x8."PRICE" < 8.0
// union select x9."COF_NAME", x9."SUP_ID", x9."PRICE", x9."SALES", x9."TOTAL"
// from "COFFEES" x9
// where x9."PRICE" > 9.0
val unionAllQuery = q1 ++ q2
// compiles to SQL (simplified):
// select x8."COF_NAME", x8."SUP_ID", x8."PRICE", x8."SALES", x8."TOTAL"
// from "COFFEES" x8
// where x8."PRICE" < 8.0
// union all select x9."COF_NAME", x9."SUP_ID", x9."PRICE", x9."SALES", x9."TOTAL"
// from "COFFEES" x9
// where x9."PRICE" > 9.0
然後他們說: Unlike union which filters out duplicate values, ++ simply concatenates the results of the individual queries, which is usually more efficient.
我覺得有沒有通過q1
和q2
產生重複。所以這是一個錯誤的查詢例子,他們提供了說明union
和++
之間的真正區別,或者我沒有看到重要的東西。你們能幫忙嗎?