Question

我有一个包含word, length, position, count and definition列的表格。我想选择word和definition但只保留一些单词的定义（基于position，count和length的条件其余的定义用空字符串替换它们。

我正在进行以下查询：

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

但我最终在列字中出现了重复的值如何在没有重复值的情况下获得此信息？

Answer 1

将反向WHERE子句添加到第二组？

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

SELECT DISTINCT似乎可以解决您的问题。

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

Answer 3

你可以使用group by子句....

Answer 4

在您的情况下使用UNION SELECT似乎毫无用处。您似乎只想在NULL时初始化定义列。定义只是输出的一部分。

使用ifnull初始化定义：

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

IFNULL 将返回定义，定义不是NULL，否则为“”。

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

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

SQL / SQlite：根据另一列的值在select语句中添加列

4 个答案: