How to Select SQL Records Matching Multiple Conditions on the Same Column?

Select records using multiple matching criteria on the same column

In SQL, selecting records that meet multiple conditions on the same column can be a challenge. Consider the following scenario:

You have a table that stores user tags. Each entry contains a contact ID, a tag, and a tag type. You only need to retrieve those contact IDs that match all the tags specified in the search form.

The query you proposed, where you use multiple conditions with the same column name, will return no results because SQL interprets it as a single condition. To overcome this problem, you can do one of two things:

Method 1: Use GROUP BY and HAVING

This method involves grouping the results by contact ID and then checking whether the number of distinct tags matching the search criteria is equal to the specified number of tags.

SELECT contact_id
FROM your_table
WHERE flag IN ('Volunteer', 'Uploaded', ...)
GROUP BY contact_id
HAVING COUNT(*) = 2 -- // 必须与WHERE flag IN (...)列表中的数量匹配

Method 2: Use connection

This method involves creating multiple connections for each condition.

SELECT T1.contact_id
FROM your_table T1
JOIN your_table T2 ON T1.contact_id = T2.contact_id AND T2.flag = 'Uploaded'
-- // 如果需要，可以添加更多连接
WHERE T1.flag = 'Volunteer'

The choice of method depends on the number of tags and the expected number of matches. For long tag lists with a small number of matches, the first approach (GROUP BY and HAVING) may be faster. However, for shorter tag lists with many matches, the second approach (concatenation) may perform better.

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