Query Statement Execution order

The order in which the query statements are executed depends on the following clauses:

1.from clauses: The order of execution is from backward, right to left

A table with a small amount of data is kept as far back as possible

2.where clauses: The execution order is bottom-up, right-to-left

Writes the criteria that can filter out the maximum number of records to the right of the WHERE clause

3.group by--The execution sequence is grouped from left to right

It is best to use where before group by to filter out unwanted records before group by

4.HAVING clause: Consuming resources

Try to avoid using it, having the result set filtered after all records have been retrieved, ordered, etc.

5.select clause: Use less * and take field names as much as possible.

During the parsing process, Oracle converts the * number to all column names by querying the data dictionary, which consumes time

6.order BY clause: Order of execution from left to right, consuming resources

Query Statement Execution order