SQL/SQlite：根據另一列的值在select語句中添加一列

Question

我有一個表格，其列word, length, position, count and definition。 我想要選擇word和definition，但只是保留一些單詞的定義（基於條件超過position，count和length），其餘定義將其替換爲空字符串。

我做下面的查詢：

SELECT word,definition FROM (SELECT word, definition, position, count FROM words 
WHERE position = 5000 AND count < 5 AND length <= 6 
AND definition IS NOT NULL 
UNION ALL 
SELECT word, "" AS definition, position, count FROM words) 
ORDER BY position ASC, count DESC 

但我最終列字重複值。
我怎樣才能得到這個沒有重複的值？

Answer 1

添加逆WHERE子句的第二套？

SELECT word,definition FROM 
(
    SELECT word, definition, position, count FROM words 
    WHERE position = 5000 AND count < 5 AND length <= 6 
    AND definition IS NOT NULL 
UNION ALL 
    SELECT word, "" AS definition, position, count FROM words 
    WHERE NOT (position = 5000 AND count < 5 AND length <= 6 
      AND definition IS NOT NULL) 
) 
ORDER BY position ASC, count DESC 

Answer 2

A SELECT DISTINCT似乎解決您的問題。

http://www.w3schools.com/sql/sql_distinct.asp

Answer 3

您可以通過子句中使用組....

Answer 4

使用UNION SELECT似乎對您的情況沒用。 看起來你只想初始化定義列，當它是NULL。 定義只是你輸出的一部分。

使用ifnull初始化定義：

SELECT word, IFNULL(definition,"") FROM words 
    WHERE position = 5000 AND count < 5 AND length <= 6 
ORDER BY position ASC, count DESC 

IFNULL將返回定義的定義不爲NULL和 「」，否則。

SQLITE ISNULL文檔 http://www.sqlite.org/lang_corefunc.html

MYSQL ISNULL文檔： http://dev.mysql.com/doc/refman/5.0/en/control-flow-functions.html#function_ifnull