SQL quiz. If query statement contains sub-queries, which wil

If query statement contains sub-queries, which will run first in the statement?

Explanation

The innermost sub-query will be executed first because other queries may be dependent on his results.

Theory

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
Rules that Sub Queries must follow
- Subqueries must be enclosed within parentheses.
- A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns.
- An ORDER BY cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY can be used to perform the same function as the ORDER BY in a subquery.
- Subqueries that return more than one row can only be used with multiple value operators, such as the IN operator.
- The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB.
- A subquery cannot be immediately enclosed in a set function.
- The BETWEEN operator cannot be used with a subquery; however, the BETWEEN operator can be used within the subquery.
Read more: Sub Queries