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SQL - Sub Queries

SQL - Sub Queries

A Subquery or Inner query or Nested query is a query within another SQL query and embedded within the WHERE clause.
A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved.
Subqueries can be used with the SELECT, INSERT, UPDATE, and DELETE statements along with the operators like =, <, >, >=, <=, IN, BETWEEN etc.

Example

SELECT sub.*
  FROM (
        SELECT *
          FROM tutorial.sf_crime_incidents_2014_01
         WHERE day_of_week = 'Friday'
       ) sub
 WHERE sub.resolution = 'NONE'
 
Let’s break down what happens:
First, the database runs the “inner query” — the part between the parentheses:
SELECT *
  FROM tutorial.sf_crime_incidents_2014_01
 WHERE day_of_week = 'Friday'
Read more: SQL - Sub Queries
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