Interview Preparation
SQL Interview: Normalisation, NULL and Query Optimisation
Design and correctness questions: the normal forms, why NULL breaks intuition, and how to answer "this query is slow, what do you do?"
#NULL Handling
NULL, unknown values and three valued logic.
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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
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
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
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...
SQL
What NULL Really Means
NULL is not zero, not an empty string and not equal to itself. Learn what it represents, how it spreads through expressions and how to handle it delib...
SQL
CASE, COALESCE, NULLIF and Conversion Functions
Conditional logic inside a query, NULL substitution, safe division and explicit type conversion - four tools that remove most procedural code from SQL...
SQL
IN, BETWEEN, LIKE and IS NULL
The four filtering shortcuts you will use constantly: set membership, ranges, pattern matching with wildcards, and the only correct way to test for NU...
SQL
AND, OR, NOT and Operator Precedence
Combining conditions correctly. Precedence between AND and OR, what NOT does to a NULL, and how to keep complex filters readable and index friendly.