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.
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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
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
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
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
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...
SQL
Multi Row Subqueries: IN, ANY and ALL
Subqueries that return a list of values, and the three operators that consume them - including the NULL trap that makes NOT IN return nothing.
SQL
Correlated Subqueries, EXISTS and NOT EXISTS
Subqueries that reference the outer row and run once per row. Learn EXISTS and NOT EXISTS, why they are NULL safe, and when correlation costs too much...
SQL
Subqueries in SELECT, FROM and WHERE: Join vs Subquery vs EXISTS
The same question answered four ways, with a straight comparison of readability, NULL safety and cost - and a rule for choosing between them.
SQL
UNION and UNION ALL
Stacking two result sets on top of each other. Learn the column compatibility rules, why UNION ALL is faster, and where ORDER BY belongs.
SQL
INTERSECT, EXCEPT and Column Compatibility
The two set operations MySQL waited longest to add: rows in both sets, and rows in one set but not the other - plus how to emulate them where they are...
SQL
WITH: Single and Multiple CTEs
A CTE names a query so the rest of the statement can read it like a table. Learn single and chained CTEs, and when a CTE beats a subquery or a view.