How is the order of fields in an index in a database determined?

The order of fields in a database index is determined by the order in which these fields are specified when creating the index. This is important because the index is built based on the sequence of field values, and searching by index is efficient only when using the prefix of this order.

For example, if an index is created on fields (A, B, C), it is effectively used for queries filtering by A, by A and B, or by A, B, and C, but not for queries only on B and C without A.

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 index efficiency.