Filtering Rows with Unique IDs in MySQL

When working with tables that contain unique identifiers, it’s often necessary to filter rows based on these IDs. In this article, we’ll explore how to achieve this in MySQL, specifically focusing on returning only the first row having a unique ID.

Understanding Unique Identifiers

Before diving into the solution, let’s first discuss what makes an identifier unique and why we might want to retrieve only the first occurrence of such an ID.

In a database table, a unique identifier is a column that uniquely identifies each row within the table. This can be an auto-incrementing primary key or any other column that can be used as a distinguishing factor between rows.

We’ll use the call_unique_id column from the provided example to illustrate this concept.

Retrieving Only the First Row with a Unique ID

The question at hand is how to return all data from only the first distinct call_unique_id. This involves finding the earliest (or smallest) value in the start_time column that corresponds to each unique call_unique_id.

We’ll explore two approaches: using a subquery and using aggregate functions.

Approach 1: Using a Subquery

The original solution provided suggests using a subquery:

select t.*
from mytable t
where t.start_time = (
    select min(t1.start_time) from mytable t1 where t1.call_unique_id = t.call_unique_id
)

Let’s break down how this works:

• The inner subquery finds the minimum start_time for each unique call_unique_id. This effectively returns the earliest record with each distinct identifier.
• The outer query then selects all columns (t.*) from the main table (mytable) where the start_time matches the smallest value found in the subquery. Since we’re looking for the first occurrence of each unique ID, this ensures that only one row per unique ID is returned.

However, there’s a potential issue with this approach: what if two or more records have the same minimum start_time for a particular call_unique_id, but are not actually the “first” record in the sense of being the earliest overall? To address this concern, we need to adjust our query slightly:

select t.*
from mytable t
where t.start_time = (
    select min(t1.start_time) from mytable t1 where t1.call_unique_id = t.call_unique_id and t1.row_num = 1
)

Here, we’ve added a condition to the inner subquery (t1.row_num = 1) that ensures we only consider records with row_num equal to 1. This effectively selects the first occurrence of each unique ID based on the earliest overall record for that ID.

Approach 2: Using Aggregate Functions

Alternatively, we can use aggregate functions like MIN() and ROW_NUMBER() to achieve the same result:

with ranked_results as (
    select call_unique_id, start_time,
           row_number() over (partition by call_unique_id order by start_time) as rn
    from mytable
)
select *
from ranked_results
where rn = 1;

In this solution, we create a temporary result set (ranked_results) that includes the call_unique_id, start_time, and a row number (rn) for each record. We use ROW_NUMBER() to assign a unique ranking to each record within each group (i.e., for each unique call_unique_id). The records are then ranked based on their start_time.

The outer query selects all columns from the temporary result set where the rn is equal to 1, effectively returning only the first row with each unique call_unique_id.

Last modified on 2023-12-23