Pseudo-Columns for Oracle databases

Pseudo- column: a pseudo-column in Oracle is like a table column (a column in a table), but it is not stored in a table, a pseudo-column can be queried from a table, but its values cannot be inserted, updated, or deleted

Common pseudo-Columns are ROWID and rownum.

use of ROWID--quickly delete duplicate records

Parse: ROWID is the detailed address of the data, through ROWID,Oracle can quickly locate the location of the specific data of a row.

ROWID can be divided into two kinds of physical ROWID and logic ROWID .

in a normal table. rowID is a physical rowID , index Organization table (IOT) of the rowID is logical rowID .

When there is a large amount of duplicate data in the table, you can use ROWID quickly delete duplicate records.

Query statement:

Select Rowid,rownum from Student

ROWNUM

ROWNUM is the ordinal of the row in the result set returned by the query, which you can use to limit the number of rows returned by the query

through testing, Rownum can only filter =1 and <n , if >m cannot filter, how to evade.

construct a temporary table from a subquery so that the pseudo-column Rownun is called a column in a staging table, and then a qualified condition uses a pseudo-column

Alias.

Using pseudo-Columns to query for data with pseudo-column numbers equal to 2

Select *from

(

Select Id,name,rownum rn from student

) Temp

where RN =2

Querying 第4-6条 data with pseudo-columns

Select *from

(

Select Temp.*,rownum rn from

(

Select *from Student

) Temp

where rownum<=6

) Where rn>=4

Pseudo-Columns for Oracle databases