The format and execution sequence analysis of simple query statements in Oracle

A simple query SQL format is as follows:

SELECT ...

From ....

[WHERE ...] --Filter Line

[GROUP by ... [Having ...] --group by groups The results of the previous where condition filtering, having a filter row group

　　

[ORDER by ...] --Sort the results

Eg: now there is a exchangetime table, the table structure is as follows

is the name empty? Type
----------------------------------------------- -------- --------------------------------
ID not NULL number (18)
System_type not NULL CHAR (1)
Time_kind not NULL number (10)
Time_name not NULL VARCHAR2 (16)
Begin_time not NULL number (10)
End_time not NULL number (10)

The query statements and results are as follows:

Sql> List

1 Select T.system_type, AVG (t.begin_time) from Exchangetime t

2 Where T.id < 10

3 GROUP BY T.system_type

4 having AVG (t.begin_time) >110000

5* ORDER BY System_type

Sql>/

System_type AVG (T.begin_time)

------------ -----------------

0 110750

1 150000

Sql> Spool Off

In the above query, where T.id < 10 first filters the records in the Exchagetime, the filtered rows are grouped by group by System_type, and the HAVING clause filters the row groups, preserving only avg (T.begin _time) >110000 Line, the last order by statement follows the System_type field to sort the results in ascending order to get the final result.

Ps:1, group by can be followed without having, but must be used in conjunction with the GROUP BY statement if there is a HAVING clause

2. Where clause can only filter a single line and cannot filter row groups, the filter row group must use a HAVING clause, such as where AVG (T.begin_time) is wrong. The row group must be filtered with having, like the above statement.

　　

The format and execution sequence analysis of simple query statements in Oracle