SQL
Subquery Basics: Scalar and Single Row Subqueries
A query inside a query. Learn where a subquery may appear, what each position requires it to return, and the error a scalar subquery raises when it re...
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SQL
Duplicate Rows from Joins and Finding Unmatched Records
Why a join multiplies rows, how to detect it, and the anti join patterns that answer "which rows have no match at all".
SQL
Multiple Joins, Join Conditions and Aliases
Building queries across four or more tables: reading a join chain, non equality conditions, join order, and keeping a long query readable.
SQL
FULL OUTER JOIN, CROSS JOIN and SELF JOIN
The three joins people meet last: keeping unmatched rows from both sides, deliberate Cartesian products, and joining a table to itself.
SQL
LEFT JOIN and RIGHT JOIN
Outer joins keep unmatched rows and fill the missing side with NULL. Learn LEFT and RIGHT JOIN, and the one mistake that silently turns an outer join...
SQL
INNER JOIN Explained
INNER JOIN keeps only rows that match on both sides. Learn the syntax, how the join condition works, and why unmatched rows silently disappear.
SQL
Multi Column Grouping and Grouping Rules
Grouping by several columns, grouping by expressions, and the totals features - ROLLUP, CUBE and GROUPING SETS - that add subtotal rows.
SQL
HAVING, and WHERE vs HAVING
HAVING filters groups after aggregation, WHERE filters rows before it. Learn when each applies, why using both is usually correct, and which is faster...
SQL
GROUP BY Fundamentals
GROUP BY collapses rows into one row per distinct key. Learn the single rule that governs the select list and how NULLs are grouped.
SQL
Aggregates, NULL and Aggregate Expressions
Aggregating an expression rather than a column: weighted totals, conditional counts and the difference between counting rows and counting facts.
SQL
COUNT, SUM, AVG, MIN and MAX
The five aggregate functions that collapse many rows into one value, the three forms of COUNT, and what each one does with NULL.
SQL
Three Valued Logic: TRUE, FALSE and UNKNOWN
SQL conditions return three results, not two. Learn the AND, OR and NOT truth tables, why NOT IN breaks on NULL, and the NULL safe comparison operator...