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PostgreSQL Expressions

An expression is composed of one or more values, operators, and PostgreSQL functions.

PostgreSQL expressions are similar to formulas. They can be applied in query statements to retrieve a result set that meets specified conditions.

Syntax

The syntax for the SELECT statement is as follows:

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

PostgreSQL expressions can be of different types, which we will discuss next.

Boolean Expression

A Boolean expression retrieves data based on a specified condition:

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

Create a COMPANY table (download COMPANY SQL file), with the following data content:

tutorialprodb# 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)

The following uses a Boolean expression (SALARY=10000) to query data:

tutorialprodb=# SELECT * FROM COMPANY WHERE SALARY = 10000;
 id | name  | age | address  | salary
----+-------+-----+----------+--------
  7 | James |  24 | Houston  |  10000
(1 row)

Numeric Expression

Numeric expressions are commonly used for mathematical operations in query statements:

SELECT numerical_expression as OPERATION_NAME
[FROM table_name WHERE CONDITION];

numerical_expression is a mathematical operation expression, with an example as follows:

tutorialprodb=# SELECT (17 + 6) AS ADDITION;
 addition 
----------
       23
(1 row)

Additionally, PostgreSQL includes some built-in mathematical functions such as:

The following example queries the total number of records in the COMPANY table:

tutorialprodb=# SELECT COUNT(*) AS "RECORDS" FROM COMPANY;
 RECORDS
---------
       7
(1 row)

Date Expression

Date expressions return the current system date and time and can be used for various data manipulations. The following example queries the current time:

tutorialprodb=# SELECT CURRENT_TIMESTAMP;
       current_timestamp       
-------------------------------
 2019-06-13 10:49:06.419243+08
(1 row)
❮ Postgresql Autoincrement Postgresql Data Type ❯