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An articulation is a mix of at least one qualities, administrators, and PostgresSQL capacities that assess to a worth. 

PostgreSQL EXPRESSIONS resemble recipes and they are written in inquiry language. You can likewise use to question the information base for explicit arrangement of information. 

Syntax

Think about the essential punctuation of the SELECT assertion as follows − 

SELECT column1, column2, columnN
FROM table_name
WHERE [CONDITION | EXPRESSION];

There are various sorts of PostgreSQL articulations, which are referenced beneath − 

PostgreSQL - Boolean Expressions

PostgreSQL Boolean Expressions bring the information based on coordinating single worth. Following is the grammar − 

SELECT column1, column2, columnN
FROM table_name
WHERE SINGLE VALUE MATCHTING EXPRESSION;

Consider the table COMPANY having records as follows − 

testdb# select * from COMPANY;
 id | name  | age | address   | salary
----+-------+-----+-----------+--------
  1 | Paul  |  32 | California|  20000
  2 | Allen |  25 | Texas     |  15000
  3 | Teddy |  23 | Norway    |  20000
  4 | Mark  |  25 | Rich-Mond |  65000
  5 | David |  27 | Texas     |  85000
  6 | Kim   |  22 | South-Hall|  45000
  7 | James |  24 | Houston   |  10000
(7 rows)

Here is the straightforward model demonstrating utilization of PostgreSQL Boolean Expressions − 

testdb=# SELECT * FROM COMPANY WHERE SALARY = 10000;

The above given PostgreSQL proclamation will create the accompanying outcome − 

 id | name  | age | address  | salary
----+-------+-----+----------+--------
  7 | James |  24 | Houston  |  10000
(1 row)

PostgreSQL - Numeric Expression

These articulations are utilized to play out any numerical activity in any question. Following is the grammar − 

SELECT numerical_expression as  OPERATION_NAME
[FROM table_name WHERE CONDITION] ;

Here numerical_expression is utilized for numerical articulation or any equation. Following is a straightforward model indicating use of SQL Numeric Expressions − 

testdb=# SELECT (15 + 6) AS ADDITION ;

The above given PostgreSQL proclamation will deliver the accompanying outcome − 

 addition
----------
       21
(1 row)

There are a few implicit capacities like avg(), whole(), tally() to perform what is known as total information counts against a table or a particular table segment. 

testdb=# SELECT COUNT(*) AS "RECORDS" FROM COMPANY;

The above given PostgreSQL proclamation will deliver the accompanying outcome − 

 RECORDS
---------
       7
(1 row)

PostgreSQL - Date Expressions

Date Expressions return the current framework date and time esteems and these articulations are utilized in different information controls. 

testdb=#  SELECT CURRENT_TIMESTAMP;

The above given PostgreSQL articulation will deliver the accompanying outcome − 

          now
-------------------------------
 2013-05-06 14:38:28.078+05:30
(1 row)