SELECT, FROM & Column Aliases

The most basic question you can ask a database is "show me the data". The SELECT statement does that. You decide which columns you want, and the FROM clause tells SQL which table to read them from.

Definition

SELECT is the DQL statement that retrieves data from one or more tables. The SELECT clause lists the columns (or expressions) to return, and the FROM clause names the table that holds the data.

A column alias is a temporary name given to a column or expression in the result, written with AS. It changes only the heading of the result — not the table.

Syntax

SELECT column1, column2, expression AS alias_name
FROM table_name;

SELECT *            -- * = every column
FROM table_name;

Example — SELECT * (all columns)

SELECT *
FROM players;

Execution order: 1️⃣ FROM players — find the table → 2️⃣ SELECT * — keep all columns.

Example — SELECT a few columns

SELECT name, team
FROM players;

How it works: FROM loads the table, then SELECT name, team keeps only those two columns, in the order you listed them. Every row is still returned — SELECT chooses columns, not rows (WHERE chooses rows).

Example — aliases and a calculated column

SELECT name AS player_name,
       runs AS season_runs,
       runs * 2 AS double_runs
FROM players;

How it works: double_runs doesn't exist in the table — SQL computes it for each row. AS gives every result column a friendly heading. The players table itself is not changed.

Tip: If an alias contains a space, wrap it: AS [Season Runs] (SQL Server) or AS "Season Runs" (standard SQL). The word AS is optional (runs season_runs also works), but writing it makes queries clearer.

Why avoid SELECT * in real projects?

SELECT * is great for exploring a table. In production code, list the columns you need:

• Performance — fewer columns means less data read from disk and sent over the network.
• Stability — if someone adds a column to the table later, your app suddenly receives extra data.
• Readability — the reader knows exactly what the query returns.

Common mistakes

• ❌ Missing comma: SELECT name team FROM players — SQL treats team as an alias for name! ✅ SELECT name, team.
• ❌ Trailing comma: SELECT name, team, FROM players → syntax error near FROM. ✅ No comma before FROM.
• ❌ Misspelt column → Invalid column name 'nmae'. ✅ Check spelling against the table.

Interview tip

Q: Does a column alias change the table? A: No. An alias only renames the column in that query's result. To rename a real column you need DDL (sp_rename in SQL Server, ALTER TABLE … RENAME COLUMN elsewhere).