>Digital-flame

SQL.How can you compare two nullable values safely, and how does IS DISTINCT FROM differ from <>?

Posted on 11.09.2026 by Stanislav_Panteleev `<>` returns **UNKNOWN** if either value is `NULL`. `IS DISTINCT FROM` provides a NULL-safe comparison: it always returns **TRUE or FALSE**, treating two NULLs as equal and one NULL as different from a non-NULL value.

SQL.How are NULL values ordered by ORDER BY, how do defaults differ between databases, and how can you control their placement?

Posted on 11.09.2026 by Stanislav_Panteleev `ORDER BY` places NULL values first or last depending on the database and sort direction.

SQL.How does NULL propagate through arithmetic expressions, and how can COALESCE and NULLIF help handle missing values and division by zero?

Posted on 11.09.2026 by Stanislav_Panteleev Arithmetic involving `NULL` produces **NULL**: ```sql 10 + NULL -- NULL 10 * NULL -- NULL ``` **COALESCE** returns the first non-NULL argument. Use it to provide a default when that default makes business sense: ```sql salary + COALESCE(bonus, 0) -- Treats a missing bonus as zero ``` **NULLIF(a, b)** returns `NULL` if the arguments are equal; otherwise, it returns `a`.

SQL.How does NULL affect join conditions, and why can filtering the right-hand table in WHERE remove unmatched rows from a LEFT JOIN?

Posted on 11.09.2026 by Stanislav_Panteleev In an equality join, `NULL` does not match any value—even another `NULL`—because the comparison evaluates to **UNKNOWN**. Join conditions match only when **TRUE**. A `LEFT JOIN` preserves unmatched left-hand rows and fills the right-hand columns with `NULL`.

SQL.How do GROUP BY and DISTINCT treat NULL values, even though NULL = NULL evaluates to UNKNOWN?

Posted on 11.09.2026 by Stanislav_Panteleev `GROUP BY` and `DISTINCT` treat NULL values as **not distinct from each other**, even though `NULL = NULL` evaluates to `UNKNOWN`. - **GROUP BY:** All NULL values in a grouping column form one group. - **DISTINCT:** Multiple NULL values in a selected column become one NULL.