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📘 How does SQL SELECT actually read?

In SQL you state WHAT result you want — which columns, from which table, matching which

4
lessons
~20 min
to learn
Adults
level
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What you’ll learn

  1. How SQL Reads a QueryExplain SQL's declarative nature and the logical clause-evaluation order that governs every query.SQL is declarative: you describe the desired result set and the planner chooses execution. Clauses are written SELECT-first but evaluated FROM, WHERE, GROUP BY, HAVING, SELECT, ORDER BY, which explains why SELECT aliases are unavailable in WHERE. Each query returns a relation, so outputs compose into further queries.
  2. Filtering Rows with WHEREWrite correct row filters using comparisons, AND/OR precedence, LIKE, IN, and BETWEEN.WHERE keeps a row only when its predicate is TRUE, filtering individual rows before grouping. AND binds tighter than OR, so parentheses are often needed; LIKE matches text with % and _, IN expresses set membership, and BETWEEN is an inclusive range. Choosing readable predicates reduces logic errors.
  3. NULLs and Three-Valued LogicHandle missing data correctly using SQL's TRUE/FALSE/UNKNOWN logic, IS NULL, and COALESCE.NULL marks unknown data and is not 0 or ''. Comparisons with NULL produce UNKNOWN, so use IS NULL / IS NOT NULL rather than = NULL, and COALESCE to supply a fallback before arithmetic. Treating NULL as zero is a common cause of wrong totals.
  4. Sorting, Limiting, De-duplicatingOrder, cap, and de-duplicate result sets predictably with ORDER BY, LIMIT, and DISTINCT.Result sets are unordered unless ORDER BY is given; NULL placement is implementation-defined (Postgres: NULLS last on ASC). LIMIT/FETCH FIRST caps rows and only yields a meaningful 'top N' after sorting, and DISTINCT removes duplicates across the whole selected row.

Questions this course answers

In what logical order does SQL evaluate these clauses?

FROM identifies rows, WHERE filters them, GROUP BY/HAVING group and filter groups, SELECT projects columns, and ORDER BY sorts last. SELECT is written first but evaluated near the end.

Why is explicitly listing columns preferred over SELECT * in production?

Naming columns makes the result set stable and self-documenting; SELECT * silently changes when the underlying table gains or loses columns.

A SELECT result set is best described as:

Every query returns a relation — named columns and a set of rows — which is why query outputs can compose into subqueries and CTEs.

WHERE keeps a row only when its predicate evaluates to:

Rows are kept only on TRUE. FALSE and UNKNOWN (e.g., comparisons involving NULL) are both excluded.

Without parentheses, `WHERE a OR b AND c` is interpreted as:

AND has higher precedence than OR, so it binds first: a OR (b AND c). Use parentheses to force the grouping you intend.

Which pattern finds values ending in 'son' using LIKE?

% matches any run of characters, so '%son' matches anything ending in son. 'son%' would match values starting with son.

Grounded in trusted sources

  • PostgreSQL documentation — SELECT and query processing, https://www.postgresql.org/docs/
  • SQL standard / ISO SQL primers on three-valued logic and NULL
  • Mode Analytics SQL tutorial — SELECT basics
  • Joe Celko, SQL for Smarties — NULL and predicate pitfalls
  • SQLite / DuckDB documentation — SELECT examples for analysts

Every Wunder lesson is built from real, reputable sources — never invented.

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