Oracle's paging query principle: 1. Use the characteristics of ORACLE's rownum field to query; 2. Query the specified number of N pieces of data; 3. Remove the M pieces of data from the specified number of N pieces of query results; 4. In web development, by passing M and N as parameters, you can get paginated results.

The operating environment of this tutorial: Windows 10 system, Oracle 11g version, Dell G3 computer.

What is the principle of oracle paging query?

The principle of paging:

1. Use the characteristics of the rownum field of ORACLE to query.

2. Query the first N pieces of data

3. Get M to N pieces of data from the query results in step 2

4. In web development Pass M and N as parameters to get paginated results.

Example: Query 20~30 records in the table, the sql statement is as follows.

select *   from( 
 select   rownum  as  pageNo,  A.*   from    tableName     A   where   rownum   <=   30)  B  where B.pageNo >=20;

Example:

--How to get the top 5 highest paid people in the employee table?

SELECT * FROM(
SELECT * FROM emp ORDER BY sal DESC)
WHERE Rownum<6

--How to obtain the information of the employee whose salary is ranked 5th in the employee table

SELECT * FROM(
SELECT * FROM emp ORDER BY sal DESC)
WHERE Rownum=5

--No content can be found in the query

--Same ROWNUM>=5 query 5 The results after the first place also failed

--This involves ROWNUM, which can only be less than but not greater than or equal to. So--an important obstacle to paging queries in Oracle database is this.

--As for the principle, ROWNUM is a pseudo column that must be arranged starting from 1 every time. So ROWNUM>=n. For example, it is not true if 1 is greater than any n (number of items). It can be found with ROWNUM=1. Because 1>=1 holds. N=2,1>=2 is not true. Therefore, only the first item can be checked for greater than or equal to. Plus less than or equal to. That’s all I can check. Therefore, this difficulty can be regarded as increasing the difficulty and obstacles of paging query.

--Method 1 The simplest query

--Articles 5 to 10.

--There is only one layer of nesting and one subquery,

SELECT * FROM
(SELECT ROWNUM rn ,e.* FROM emp e WHERE ROWNUM<=10)
WHERE rn>=5;

--This is actually transformed from the following method.

--That is, the new table obtained by the subquery is queried again.

--The key step is ROWNUM rn.

--And don't forget e.*, otherwise there will be no complete data.

SELECT * FROM (SELECT ROWNUM rn, e.*
          FROM emp e
         WHERE ROWNUM <= 10) table_1
 WHERE table_1.rn>= 5;

--The above is not sorted, the following is sorted

--Because it must be sorted first before querying, the larger the table, the slower the efficiency.

--Method 2

SELECT * FROM
(SELECT ROWNUM rm ,e.* FROM emp e order by sal DESC )
WHERE rm>=5 AND rm<=10

--The following is the method of using analytical functions;

--Method 3

SELECT * 
FROM (SELECT emp.*,
   ROW_NUMBER () OVER (ORDER BY sal DESC) rank 
    FROM emp)   
WHERE rank >=6 AND rank<=10;

--But our actual query Looking at the results, it turns out they are completely different.

--Checked manually. The results of the analytical function method are correct. Don't understand.

--Can anyone explain it?

--Here comes a meaningless

SELECT * FROM 
(SELECT e.*, ROWNUM AS rn from 
( SELECT * FROM emp ORDER BY sal DESC) e
)ee 
WHERE ee.rn>=5 AND ee.rn<=10

--Method 4 (massive data query, such as Baidu, Tmall query)

SELECT * FROM(
SELECT e.* ,ROWNUM rn
FROM (
SELECT *
FROM emp
ORDER BY sal DESC
) e
WHERE  ROWNUM<=10
)WHERE rn>=6;

--Decomposition steps

--The first step:

SELECT * FROM emp ORDER BY sal DESC

--The second step:

SELECT e .*,ROWNUM rn FROM
(SELECT * FROM emp ORDER BY sal DESC) e

--The third step:

SELECT e .*,ROWNUM rn FROM
(SELECT * FROM emp ORDER BY sal DESC) e
WHERE ROWNUM<=10

--The fourth step:

SELECT * FROM(
SELECT e .*,ROWNUM rn FROM
(SELECT * FROM emp ORDER BY sal DESC) e
WHERE ROWNUM<=10)
WHERE rn>=5

--Wrong guess.

SELECT e .*,ROWNUM rn FROM
(SELECT * FROM emp ORDER BY sal DESC) e
WHERE rn<=10

--To create an alias, you must query the data table.

--The query conditions are judged by conditions that do not yet exist, so they naturally fail.

--Method five, idle and boring usage

WITH ee AS (
  SELECT e.*, ROWNUM rn FROM
 ( SELECT * FROM emp ORDER BY sal DESC )e
  )
  SELECT ee.*
  FROM ee
  WHERE ee.rn>=5 AND ee.rn<=10

Recommended tutorial: "Oracle Video Tutorial"

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