SQL Language Reference

Set operators Union, union All, INTERSECT, and minus
These three operators have the same precedence, from left to right, unless there are parentheses

Library single table operation, single-Library multi-table operation, multi-Library multi-table operation (use to Dblink)

Transcendental function transcendental functions, such as: Sin,cos,tan, etc.

Functions are similar to operators, they all maintain data items and return results, and differ from operators in parameter formats

Functions that do not have any parameters are similar to pseudo-columns

Pseudo-Columns return different values per row

Functions that do not have any parameters return the same value for each row

They convert each other.

Select Nls_charset_name (2) from dual

Select nls_charset_id (' We8dec ') from dual

The following three column names are not the same

Select Length (' Canada ') from dual length (' Canada ') as column name and uppercase

Select Length (' Canada ') Woshi from dual Woshi as column name and large print single-byte character set returns 6 if the double-byte database character set returns 12

Select Length (' Canada ') "Woshi" from dual Woshi as the column name is output (in quotation marks what is what) single-byte character set returns 6 if the double-byte database character set returns 12

Select CONCAT (CONCAT (last_name, "job category is"), job_id) "Job"
From Employees
WHERE employee_id = 152;

Concat This function was equivalent to the concatenation operator (| |).

comparison function

Select Greatest (' HARRY ', ' Harriot ', ' HAROLD ') ' greatest ' from DUAL;
Select Greatest (1, ' 3.925 ', ' 2.4 ') "greatest" from DUAL;

Environment and identity functions

Select Userenv (' Terminal ') from dual

Select Userenv (' Sid ') from dual

Select User,uid from dual

SQL Language Reference