SQL / SQlite：根据另一列的值在选择语句中添加一列

Question

I have a table with the columns word, length, position, count and definition . I want to make a select of word and definition but just keeping the definition of some words (based on a condition over position , count and length ) and the rest of definitions replace them by an empty string.

I'm doing the following query:

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

But I end up with duplicated values in the column word.
How can I get this without duplicated values?

Answer 1

Add the inverse WHERE clause to your second set?

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 seems to solve your problem.

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

Answer 3

您可以使用group by子句。

Answer 4

Use of UNION SELECT seems to be useless in your case. It seems that you want only to initialize definition column when it is NULL . Definition is only part of your outputs.

Use a ifnull to initialize definition :

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

IFNULL will return definition is definition is not NULL and "" otherwise.

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

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