Properties of a Sub-Query
- A sub-query must be enclosed in the parenthesis.
- A sub-query must be put on the right hand of the comparison operator, and
- A sub-query cannot contain an ORDER BY clause, however sub-query can use ORDER BY when used with TOP clause.
Types of Sub-query
- Single-row sub-query, where the sub-query returns only one row.
- Multiple-row sub-query, where the sub-query returns multiple rows, and
- Multiple column sub-query, where the sub-query returns multiple columns
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