SQL

SQL (Structured Query Language) is a standard language used to operate relational databases. It includes operations such as database creation, retrieving and modifying data, granting privileges to data.

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Correlated Subquery

A correlated subquery is a type of nested subquery that uses columns from the outer query in its WHERE clause. While a simple subquery is evaluated only once for each table, a correlated subquery is evaluated once for each row.

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Understanding Subqueries and Their Types

A subquery – that is, a query within a query – can be of various flavors in Oracle. Subqueries may be simple or correlated, nested or scalar. A closer look at the types of subqueries and their purpose.

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Select Rows With Max Value

Solution for selecting rows based on the maximum value of a specific table column, such as date. See it in action: script to list the latest order details of every customer’s orders.

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Why the SQL WITH clause is not exactly like a function definition

SQL WITH clause appears similar in concept to a function definition in procedural code, but it differs in an important way in older Oracle versions. The error ORA-32035: unreferenced query name defined in WITH clause informs you about this difference.

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Write Smarter Queries with the SQL WITH Clause

SQL WITH clause, or the subquery factoring clause, makes queries more efficient and readable. When a subquery is being processed multiple times, WITH lets you factor it out, give it a name and then reference the name wherever needed in the query.

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Simple CASE vs. Searched CASE

The CASE construct in Oracle has two variants – the simple CASE and the searched CASE. Here’s a closer look to compare them in structure and functionality.

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The Difference Between DECODE and CASE

Both DECODE and CASE statements in Oracle are used for IF-THEN-ELSE conditional operations, but there are distinct differences in their power and behavior.

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Selecting ODD or EVEN rows from a table

Question: How can I select only the even/odd rows from an Oracle table? Answer: Talking of “even or odd rows” is meaningless in Oracle until you have ordered the rows.

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LEVEL Pseudocolumn in Hierarchical Queries

LEVEL is a pseudocolumn (i.e. not a real column in the database but available in a query), which has a special function in hierarchical queries – it returns the position of any row in the hierarchy.

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Hierarchical Queries: A QuickStart Guide

A hierarchical query is one that works on data with a tree relationship. Oracle provides specialized keywords that work with hierarchical queries, such as START WITH…CONNECT BY, PRIOR and SYS_CONNECT_BY_PATH.

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