How Can I Find the First Missing Value in a Sequential Counter Column Using SQL?

Use SQL to find gaps in running counters

When working with data tables that contain running counter columns, you may need to identify missing sequential values ​​in these counters. In SQL, this task can be accomplished efficiently using a combination of set theory and sorting techniques.

To determine the first gap in the counter column, we use the NOT EXISTS operator in conjunction with a subquery to filter out rows with consecutive values. This ensures that we isolate rows with missing sequences. We then isolate the first discovered gap by sorting the rows in ascending order using the ORDER BY clause.

MySQL and PostgreSQL:

<code class="language-sql">SELECT  id + 1
FROM    mytable mo
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    mytable mi 
        WHERE   mi.id = mo.id + 1
        )
ORDER BY
        id
LIMIT 1</code>

SQL Server:

<code class="language-sql">SELECT  TOP 1
        id + 1
FROM    mytable mo
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    mytable mi 
        WHERE   mi.id = mo.id + 1
        )
ORDER BY
        id</code>

Oracle:

<code class="language-sql">SELECT  *
FROM    (
        SELECT  id + 1 AS gap
        FROM    mytable mo
        WHERE   NOT EXISTS
                (
                SELECT  NULL
                FROM    mytable mi 
                WHERE   mi.id = mo.id + 1
                )
        ORDER BY
                id
        )
WHERE   rownum = 1</code>

ANSI (all databases supported, but less efficient):

<code class="language-sql">SELECT  MIN(id) + 1
FROM    mytable mo
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    mytable mi 
        WHERE   mi.id = mo.id + 1
        )</code>

DBMS with sliding window function:

<code class="language-sql">SELECT  -- TOP 1
        -- Uncomment above for SQL Server 2012+
        previd + 1
FROM    (
        SELECT  id,
                LAG(id) OVER (ORDER BY id) previd
        FROM    mytable
        ) q
WHERE   previd <> id - 1
ORDER BY
        id
-- LIMIT 1
-- Uncomment above for PostgreSQL</code>

By leveraging these techniques, you can effectively identify missing values ​​in sequential counter columns in SQL to perform data analysis and maintenance tasks efficiently.

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