Question 1

Select one column

Accepted Answer

SELECT name fetches only the name column from every row. The result has one column and three rows - one per employee. Selecting a single column returns just that column's values, not the whole row.

Question 2

Arithmetic expression in SELECT

Accepted Answer

You can write arithmetic expressions directly in SELECT. Here quantity * unit_price evaluates to 3 * 15.00 = 45.00 for the row where id = 1. The result column has no name unless you add an alias with AS. Expressions can use +, -, *, /, and parentheses like in any programming language.

Question 3

Column order in SELECT

Accepted Answer

The SELECT clause controls the order of columns in the result - not the table's physical column order. Because price is listed before name, the result row is 10.00 | Widget, not Widget | 10.00. This trips people up when they assume results mirror the table definition.

Question 4

What SELECT * returns

Accepted Answer

SELECT * returns every column in the order they appear in the table's CREATE TABLE definition. There is no sorting, no NULL filtering, and no extra columns. Avoid it in production queries - adding a new column later will silently change what your query returns.

Question 5

Selecting a constant value

Accepted Answer

Selecting a literal repeats that value for every row in the source table - one output row per source row. With 3 employees the result has 3 rows, all with the value 'active' in the status column. This pattern is useful for tagging all rows from a table before combining them with UNION.

Question 6

Column alias with AS

Accepted Answer

AS renames the result column. The value 90000 * 1.1 = 99000.0 is returned under the column header new_salary. The alias affects only the output column name, not the calculation itself. Aliases are most useful for giving computed expressions a readable name.

Question 7

LIMIT rows returned

Accepted Answer

LIMIT N restricts the result to at most N rows. Without ORDER BY, the database may return any N rows - in practice most engines return the first N in storage order. Here the result is Alice and Bob. Always pair LIMIT with ORDER BY when you need a predictable subset.

Question 8

SELECT * vs named columns

Accepted Answer

SELECT * always returns all columns defined on the table, regardless of which column you filter on in WHERE. The WHERE clause decides which rows appear, not which columns. All three columns - id, username, email - show up in the result.

Question 9

Alias without AS keyword

Accepted Answer

The AS keyword is optional when defining a column alias. Writing the alias name directly after the expression works in all major SQL databases. The = and : forms are not valid standard SQL (though = is specific to some T-SQL syntax). Using AS explicitly is recommended because it makes the alias easier to spot when reading the query.

Question 10

Selecting a numeric literal

Accepted Answer

Mixing a column reference with a literal produces the column value alongside the literal for each row. Every row gets the constant 0 in the discount column, just as it gets its own name value. This is handy when you want to add a placeholder column before a join or union fills it in with real data.