Shulou(Shulou.com)05/31 Report--

This article mainly shows you "how to use SELECT in SQL", the content is easy to understand, clear, hope to help you solve doubts, the following let the editor lead you to study and learn "how to use SELECT in SQL" this article.

1. About *

* is not recommended. If some columns are not required, it will degrade performance.

Using * will force the use of a data dictionary and convert it to a specific column name

It is recommended that you use specific column names

two。 Data dictionary

User_ Table

Table_ name column, etc.

3. Pseudo column rowid rownum currval nextval

Is not actually stored in the table.

Cannot insert or delete operation

Rownum: reflects the position of each row in the result set of the current query

Select ename from emp where rownum

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