如何避免SQL查询结果重复？保留单列全部非重复值的技术问询

Hey there! I get exactly what you're aiming for—you want to scrub duplicate records for the same object, but hang onto all unique values from one specific column that has varying entries. Right now, using an equality (=) operator in your State table join only pulls back a single row tied to the current state, which isn't meeting your needs. Let's work through a couple of SQL fixes:

1. Aggregate Unique State Values into a Single Column #

If you want one row per object with all unique state values bundled into a single readable column, use an aggregation function matched to your database:

For SQL Server: #

SELECT 
    mt.object_id,
    mt.column1, -- Swap in your actual non-duplicate columns
    mt.column2,
    STRING_AGG(DISTINCT s.state_value, ', ') AS all_unique_states
FROM YourMainTable mt
JOIN State s ON mt.object_id = s.object_id
GROUP BY mt.object_id, mt.column1, mt.column2;

For MySQL: #

SELECT 
    mt.object_id,
    mt.column1,
    mt.column2,
    GROUP_CONCAT(DISTINCT s.state_value SEPARATOR ', ') AS all_unique_states
FROM YourMainTable mt
JOIN State s ON mt.object_id = s.object_id
GROUP BY mt.object_id, mt.column1, mt.column2;

This groups rows by the consistent columns of your main object, then collects all unique state values into one comma-separated string.

2. Keep Rows for Each Unique State (No Redundant State Entries) #

If you'd rather have a separate row for each unique state (but no duplicate state rows for the same object), use DISTINCT to filter out redundant state entries:

SELECT DISTINCT
    mt.object_id,
    mt.column1,
    mt.column2,
    s.state_value
FROM YourMainTable mt
JOIN State s ON mt.object_id = s.object_id;

This will show each unique state for an object once, while repeating the other consistent object columns (since those values don't vary across duplicates anyway).

Why Your Original = Join Didn't Work #

When you use = to join on a specific state value, you're filtering the State table to only rows that match that exact state. That's why you only get one result—you're explicitly limiting to a single state instead of pulling all possible states tied to the object.

内容的提问来源于stack exchange，提问作者M Sami Ur Rehman