Literal Character String in SELECT statement

• A literal is a character, a number or a date included in the SELECT statement list.
• Date and character literal values must be enclosed within single quotation marks.
• Each character string is output once for each row returned.

Using Literal Character String:

The Example above displays the names and job codes of all employees. The column has the heading Employee Details. Notice the spaces between the single quote marks in the select statement. The spaces improves the readability of the output. 

Concatenation Operator in SELECT statement

A concatenation operator:

• Concatenates columns or character strings to other columns.
• is represented by two vertical bar ( || )
• Creates a resultant column that is character expression.

Using the Concatenation Operator:

In the above example ename and job are concatenated, and they are given alias Employees. Notice that the ename and job code are combined to make a single output column.

The AS keyword before the alias name makes the SELECT clause easier to read.

ALIAS - Column Aliases in SELECT statement

A column alias:

• Renames a column heading.
• Is useful with calculations.
• AS keyword is optional between the column name and alias.
• Requires double quotation marks if it contains spaces or special characters or is case sensitive

Using Column Aliases (with example)

Operator Precedence in SELECT statement

*      /     +     -

• Multiplication and division take priority over addition and subtraction
• Operators of the same priority are evaluated from left to right 
• Parentheses are used to force prioritized evaluation and to clarify statements

Operator Precedence with example

SELECT ename,sal,12*sal+100 

FROM emp;

SELECT ename,sal,12*(sal+100)

FROM emp;

In the above examples parentheses makes a lot of differences in arithmetic calculation.