How to avoid duplicate data when INNER JOIN joins tables in the database

When performing SQL queries, we often need to join multiple tables to obtain more comprehensive data. However, when using INNER JOIN to join tables, you sometimes encounter duplicate data problems, which may lead to inaccurate query results or reduced performance.

INNER JOIN is a commonly used connection method in relational databases. It can match data in two or more tables according to specified conditions and return a data set that meets the conditions. However, when there is duplicate data in the joined tables, INNER JOIN may cause redundant data in the results, affecting query accuracy and performance.

Use the DISTINCT keyword

The DISTINCT keyword is used to remove duplicate rows from query results and retain unique rows. By adding the DISTINCT keyword in the SELECT statement, you can avoid the problem of duplicate data when INNER JOIN joins tables.

The DISTINCT keyword will deduplicate the results of the entire SELECT statement, so if the query results contain multiple columns, you need to ensure that all column combinations are unique.

The DISTINCT keyword may have a certain impact on query performance, especially when the amount of data in the joined tables is large. Therefore, there is a trade-off between accuracy and performance of query results when using the DISTINCT keyword.

Use subquery

By using subqueries, the results of the join table can be processed as a temporary table to avoid the problem of duplicate data. Specifically, we can first query the unique values ​​in the table that needs to be connected, and then connect with the main query to avoid the generation of duplicate data.

Example using subquery

SELEC T column1, column2
FROM table1
INNER JOIN (
  SELEC T DISTINCT column3
  FROM table2
) AS subquery
ON table1.column4 = subquery.column3;

Use the GROUP BY clause

The GROUP BY clause is used to group the result set by specified columns, thereby combining duplicate data into a single row. By using the GROUP BY clause while connecting tables with INNER JOIN, you can avoid the problem of duplicate data.

The GROUP BY clause must include all columns in the SELECT statement, or use aggregate functions to process columns that are not included in the GROUP BY clause.

Using the GROUP BY clause may have a certain impact on query performance, especially when the amount of data in the connected tables is large. Therefore, when using the GROUP BY clause, you need to balance the accuracy of query results and performance.

Duplicate data is a common problem when joining tables using INNER JOIN. You can use the DISTINCT keyword, use subqueries, use GROUP BY clauses and other methods to avoid the generation of duplicate data. At the same time, some precautions and optimization suggestions are also put forward to help developers better deal with duplicate data problems that may occur when INNER JOIN joins tables. By rationally selecting and using these methods, we can improve the accuracy and performance of queries to better meet business needs.