[MySQL] query filter on, where, having

Execution sequence:First, on, where, and having. Usage:On: On is used for filtering when multi-table join queries are performed. On first, it filters out data that does not meet the query conditions, thus reducing the number of data operations during the Query Process. Where: after a temporary table is queried, it is executed before data computing. Because the data has not been grouped and the row and column selection operations Where cannot be applied(1) Use the column alias, for example, select name as N from TB where n = 'xiaohua'; (2) use the statistical method (Min, Max, sum, etc ), for example, select *, ID from TB where count (ID) <2; having: The having operation is meaningful only when group by is used. Having is executed after data computing. Summary:(1) Multi-table join queries do not require on filtering. (2) In a single table query, if the select field does not involve the calculation process, the where and having results will be the same after execution.