How Can SQL Identify the First Missing Value in a Running Counter Column?

Use SQL to find gaps in running counters

SQL provides a powerful mechanism to find missing values ​​in a sequence of running counters in a table.

To determine the first missing value in the run counter column (for example, values ​​1, 2, 4, and 5 are present, but 3 is missing), you can use the following SQL technique:

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 SQL (least 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>

Systems that support sliding window functions:

<code class="language-sql">SELECT previd + 1
FROM (
    SELECT id,
    LAG(id) OVER (ORDER BY id) previd
    FROM mytable
) q
WHERE previd < id
LIMIT 1;</code>

By using any of these SQL techniques, you can effectively identify missing gaps in run counter columns in SQL tables, regardless of which database system you are using.

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