SQL Joining Multiple Tables 3

Question 1

To find customers who have NEVER placed an order, you LEFT JOIN Orders and then filter how?

Accepted Answer

WHERE Orders.id IS NULL

Answer

Unmatched rows have NULL in the order columns, so IS NULL isolates them.

Question 2

When joining three tables A, B, and C, how many ON conditions are typically needed?

Accepted Answer

Two

Answer

Each additional joined table generally needs one more ON condition to link it.

Question 3

Which comparison is INVALID for matching NULLs in a join condition?

Accepted Answer

column = NULL

Answer

NULL is never equal to anything, so '= NULL' never matches; use IS NULL.

Question 4

A RIGHT JOIN of Employees and Departments keeps all rows from which table?

Accepted Answer

Departments

Answer

RIGHT JOIN preserves all rows from the right (second) table, Departments.

Question 5

Why are table aliases especially useful in multi-table joins?

Accepted Answer

They shorten references and disambiguate same-named columns

Answer

Aliases make queries readable and resolve ambiguity when columns share names.

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