Interview Preparation
SQL Interview: Joins, Keys and Constraints
The questions asked in almost every SQL interview, with short answers you can actually say out loud and the follow up the interviewer will ask next.
#Joins
Combining rows from several tables.
10 notes tagged
SQL
Intermediate Practice: Joins and Aggregation
Fifteen exercises combining joins, grouping and having, each with the solution and a note on the trap it was designed to expose.
SQL
Scans, Seeks, Join Strategies and Sorting
What the engine actually does: full scans versus index seeks, nested loop and hash joins, and why a sort or a temporary table appears in your plan.
SQL
Many to Many Relationships and Junction Tables
Two tables that both have many of each other need a third table. Learn the junction table pattern, its key design, and how to query and count across i...
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.
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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".
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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.
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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.
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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.