Understanding Duplicate Counts from Two Tables

In this article, we will explore a common use case where you need to display duplicate counts from two tables. One table has a column with a separate value for each occurrence of the duplicate value, while another table is used as a reference table to get the count of duplicates.

Background

Suppose we have two tables: Office_1 and Office_2. We want to get the duplicate counts from these tables based on the values in the OP column. However, instead of using a single column with all occurrences, we have separate columns for each occurrence ('Office_1 Come COUNT' and 'Office_1 Go COUNT').

Query Approach

To solve this problem, we can use a combination of subqueries and union to get the desired output.

First Table (Office_1)

The first query is used to get the count of duplicates from Office_1. The query uses the following approach:

• Selects all columns (EID, 'Office_1 Come COUNT', and 'Office_1 Go COUNT') from a subquery.
• In the subquery, we use the SUM aggregation function with conditional expressions to count the occurrences of 'come' and 'go'.
• We filter the results based on the date range ('2022-01-16' to '2022-01-18') to get only the records within that time frame.
• Finally, we group by the UID column.

Second Table (Office_2)

The second query is used to get the count of duplicates from Office_2. The approach is similar to the first query, but we use different columns ('Office_2 Come COUNT' and 'Office_2 Go COUNT') as our results.

Combining Results Using UNION

To combine the results from both tables into a single output, we can use the UNION operator. The basic syntax of the union is:

SELECT column1, column2 FROM table1
UNION
SELECT column1, column2 FROM table2;

In our case, we want to select all columns (EID, 'Office_1 Come COUNT', and 'Office_1 Go COUNT') from the first subquery and all columns (EID, 'Office_2 Come COUNT', and 'Office_2 Go COUNT') from the second subquery.

However, since we want to get only one row per UID value, we need to use a different approach. We can create a temporary table that combines the results of both queries based on the UID column, and then use this temporary table in our final query.

Creating a Temporary Table

We can create a temporary table using the following SQL statement:

CREATE TEMPORARY TABLE temp_table AS
SELECT UID, 'Office_1 Come COUNT' as come_count, 'Office_1 Go COUNT' as go_count
FROM Office_1 WHERE DATE >= '2022-01-16' AND DATE <= '2022-01-18'
GROUP BY UID;

CREATE TEMPORARY TABLE temp_table_2 AS
SELECT UID, 'Office_2 Come COUNT' as come_count, 'Office_2 Go COUNT' as go_count
FROM Office_2 WHERE DATE >= '2022-01-16' AND DATE <= '2022-01-18'
GROUP BY UID;

Final Query

Now we can use the temporary tables to get our final output. We will use a subquery to select all rows from the temporary table temp_table, and another subquery to select all rows from the temporary table temp_table_2.

SELECT *
FROM (
  SELECT UID, 'Office_1 Come COUNT' as come_count, 'Office_1 Go COUNT' as go_count FROM temp_table
  UNION ALL
  SELECT UID, 'Office_2 Come COUNT' as come_count, 'Office_2 Go COUNT' as go_count FROM temp_table_2
) AS combined_table;

This will give us the final output with all occurrences of 'come' and 'go' from both tables.

Explanation of UNION ALL

Note that we use UNION ALL instead of just UNION. This is because we want to include duplicate rows in our final result. If we used only UNION, we would get only one row per group, even if there are multiple occurrences of the same value in both tables.

Conclusion

In this article, we explored a common use case where you need to display duplicate counts from two tables with separate columns for each occurrence. We provided an example query that uses subqueries and union to combine the results into a single output.

The approach can be adapted to different scenarios depending on your database schema and requirements.

Last modified on 2023-10-20