What are the properties and different Types of Sub-Queries?

1.     Properties of Sub-Query

1.     A sub-query must be enclosed in the parenthesis.

2.     A sub-query must be put in the right hand of the comparison operator, and

3.     A sub-query cannot contain an ORDER-BY clause.

4.     A query can contain more than one sub-query.

2.     Types of Sub-Query

1.     Single-row sub-query, where the sub-query returns only one row.

2.     Multiple-row sub-query, where the sub-query returns multiple rows,. and

3.     Multiple column sub-query, where the sub-query returns multiple columns

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