What is the significance of the sequence of fields when creating an index in a database?

The order of fields when creating a composite index in a database is critically important because it affects search efficiency and index usability.

The index is built based on the sequence of fields, and searching by the index is effective if queries use the fields in the same order, starting with the first field of the index.

For example, if an index is created on fields (A, B, C), then:

• Searching by A or by A and B or by A, B, and C will use the index effectively.
• Searching only by B or by B and C without A will not be able to fully utilize this index.

Therefore, when designing an index, it is important to consider which fields are most frequently used in WHERE conditions and in what order, to maximize query performance.