Composite Index (referenced by Jiang min)

Union Index usage Summary: 1): the Union index can be used if the first or all of the Union indexes appear in the query conditions. 2): as long as the conditions in the condition column are connected together, in this example: last_name = '1' and first_name = '1' and first_name = '1' and last_name = '1', both front and back use the Union index. 3): the first column of the Union index does not appear in the query condition, and the second or third column of the Union index does not use the Union Index query. summary of the application of a single column index: 1): As long as an index column appears in a condition column, index query can be used no matter where it is located.