University:
American Baptist College
Course:
CSCI 1534 | Data Analysis and Visualization
Academic year:

2023
Views:

91

Pages:

2
Author:

Muthia M.

Subquery A Common Table Expression is a named temporary result set. You create a CTE using a WITH query, then reference it within a SELECT, INSERT, UPDATE, or DELETE statement. If you use a CTE, you don’t need to create and drop a table. You can simply reference the temporary result set created by the WITH query, like you see below for the example: A Subquery or Inner query or a Nested query is a query within another SQL query and embedded within clauses, most commonly in the WHERE clause. It is used to return data from a table, and this data 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. There are a few rules that subqueries must follow 1. Subqueries must be enclosed within parentheses. 2. 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. 3. An ORDER BY command cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY command can be used to perform the same function as the ORDER BY in a subquery. 4. Subqueries that return more than one row can only be used with multiple value operators such as the IN operator. 5. The SELECT list cannot include any references to values that evaluate to a BLOB, ARRAY, CLOB, or NCLOB. 6. A subquery cannot be immediately enclosed in a set function. 7. The BETWEEN operator cannot be used with a subquery. However, the BETWEEN operator can be used within the subquery. For example: SELECT * FROM CUSTOMERS WHERE ID IN (SELECT ID FROM CUSTOMERS WHERE SALARY > 4500); SELECT sub.* FROM (SELECT * FROM tutorial.sf_crime_incidents_2014_01 WHERE day_of_week = 'Friday' ) sub WHERE sub.resolution = 'NONE'