SQL WHERE: Difference between revisions

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<font color=darkkhaki>TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Inequality%20conditions,an%20inequality%20condition%3A</font>
<font color=darkkhaki>TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Inequality%20conditions,an%20inequality%20condition%3A</font>
==Existence Conditions==
<code>EXISTS</code>


==Range Conditions==
==Range Conditions==
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<font color=darkkhaki>TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Using%20regular%20expressions</font>
<font color=darkkhaki>TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Using%20regular%20expressions</font>
==<tt>NULL</tt> in Conditions==
==<tt>NULL</tt> in Conditions==
<font color=darkkhaki>TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Null%3A%20That%20Four-Letter%20Word</font>
To test whether a value is <code>NULL</code> use the <code>IS</code> operator, not <code>=</code>:
Also see: {{Internal|SQL#SQL_NULL|<tt>NULL</tt>}}
 
=<span id='CASE'></span>SQL Conditional Logic=
 
SQL conditional logic is the ability of SQL statements to take one of several paths during execution, depending on the value of an expression. The SQL has syntax that provides IF/ELSE conditional logic, except that it does not use the <code>IF</code> keyword but <code>CASE</code>. These expressions can be used in the <code>SELECT ... WHERE</code> clause, but also with conditional <code>SELECT</code>, <code>[[SQL_INSERT#Conditional_INSERT|INSERT]]</code>, <code>[[SQL_UPDATE#Conditional_UPDATE|UPDATE]]</code> and <code>[[SQL_DELETE#Conditional_DELETE|DELETE]]</code>.
 
All major databases have built-in function that mimic IF/ELSE statements. However, the <code>CASE</code> expressions should be preferred instead, because <code>CASE</code> is part of the SQL92 standard and thus available on all standard-compliant servers. <code>CASE</code> has been built into the grammar of the <code>SELECT</code>, <code>[[SQL_INSERT#Conditional_INSERT|INSERT]]</code>, <code>[[SQL_UPDATE#Conditional_UPDATE|UPDATE]]</code> and <code>[[SQL_DELETE#Conditional_DELETE|DELETE]]</code> statements, and in the <code>WHERE</code> clause.
 
There are two different types of <code>CASE</code> syntax: [[#Searched_CASE_Expressions|searched CASE expressions]] and [[#Simple_CASE_Expressions|simple CASE expressions]].


==Searched <tt>CASE</tt> Expressions==
<syntaxhighlight lang='sql'>
<syntaxhighlight lang=sql>
SELECT * FROM person where name IS NULL;
CASE
    WHEN <conditional_expression_1> THEN <expression_1>
    WHEN <conditional_expression_2> THEN <expression_2>
    ...
    [ELSE <default_expression>]
END
</syntaxhighlight>
</syntaxhighlight>


When the <CODE>CASE</CODE> expression is evaluated, the <code>WHEN</code> conditional expressions are evaluate in order from top to bottom. As soon as one of the conditional expressions is evaluated to TRUE, the corresponding expression is evaluated and returned. If none of the <code>WHEN</code> conditional expressions evaluate to true, then expression introduced by <code>ELSE</code> is evaluated and returned. If the <code>CASE</code> does not have an <code>ELSE</code>, <font color=darkkhaki>then <code>NULL</code> is returned</font>.
The following syntax is incorrect, and while will not generate a syntax or runtime error, it will produce invalid results, it will return no rows:
 
<syntaxhighlight lang='sql'>
All returned expressions (<code><expression_1></code>, <code><expression_2></code>, ... <code><default_expression></code>) must evaluate to the same type.
SELECT * FROM person where name = NULL;
 
This is an example of using <code>CASE</code> in <code>SELECT</code>:
<syntaxhighlight lang=sql>
SELECT name,
  CASE
    WHEN active = 1 THEN 'ACTIVE'
    ELSE 'INACTIVE'
  END activity_level
FROM person
</syntaxhighlight>
</syntaxhighlight>


<font color=darkkhaki>TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Null%3A%20That%20Four-Letter%20Word</font>
Also see: {{Internal|SQL#SQL_NULL|<tt>NULL</tt>}}


This is an example of using <code>CASE</code> in <code>SELECT ... WHERE</code>:
=<span id='CASE'></span>SQL Conditional Logic=
 
{{Internal|SQL_Conditional_Logic#Overview|SQL Conditional Logic}}
<code>CASE</code> can also be used in:
* Conditional INSERT.
* Conditional UPDATE.
* Conditional DELETE.
 
==Simple <tt>CASE</tt> Expressions==
 
==<tt>CASE</tt>==
 
 
 
<code>CASE</code> provides an IF/ELSE conditional logic that can be used in <code>WHERE</code> clauses.
 
<syntaxhighlight lang=sql>
CASE <expression>
    WHEN <result_1> THEN <expression_1>
    WHEN <result_2> THEN <expression_2>
    ...
    [ELSE <default_expression>]
END
</syntaxhighlight>
 
A simple IF/ELSE:
 
<syntaxhighlight lang=sql>
CASE <expression_that_evaluates_to_true_or_false> WHEN TRUE THEN <if_expression>
ELSE <else_expression>
END
</syntaxhighlight>
 
Usage example:
<syntaxhighlight lang=sql>
CASE array_length(person.skills, 1) > 0 WHEN TRUE THEN ('skill_we_search_for' = ANY(person.skills))
ELSE 'key_we_search_for' = ANY(general.skills)
END
</syntaxhighlight>
 
<font color=darkkhaki>TO PROCESS: https://www.geeksforgeeks.org/sql-conditional-expressions/</font>

Latest revision as of 21:06, 30 May 2024

Internal

TODO

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html

Overview

The WHERE clause is the mechanism for filtering out unwanted data from the result set. The WHERE clause can be used with SELECT, UPDATE and DELETE, but not with INSERT. The WHERE clause may contain an arbitrary number of filter conditions separated by AND, OR and NOT operators. The filter conditions may be optionally grouped together with parentheses. 

[...] WHERE <filter_condition> AND|OR <filter_condition> ...


SELECT * FROM person WHERE person.name = 'Alice' AND (person.eye_color = 'blue' OR person.eye_color = 'black');

Using Parentheses

If the WHERE clause includes three or more conditions combined with AND, OR or NOT, you should use parentheses to make your intent clear.

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Using%20Parentheses

Filter Conditions

A condition is made up of one or more expressions, combined with one or more operators. An expression can be any of the following:

  • A number
  • A column in a table or a view
  • A string literal
  • A built-in function such as CONCAT()
  • A subquery
  • A list of expressions such as ('A', 'B', 'C')

The operators used within conditions include:

  • Comparison operators: =, !=, <, >, <>, LIKE, IN, BETWEEN.
  • Arithmetic operators: +, -, /, *

Equality Conditions

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Equality%20Conditions

Inequality Conditions

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Inequality%20conditions,an%20inequality%20condition%3A

Existence Conditions

EXISTS

Range Conditions

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Range%20Conditions

The BETWEEN Operator

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=The%20between%20operator

String Ranges

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=String%20ranges

Membership Conditions

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Using Subqueries

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Using%20subqueries

Matching Conditions

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Matching%20Conditions

Using Regular Expressions

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Using%20regular%20expressions

NULL in Conditions

To test whether a value is NULL use the IS operator, not =: 

SELECT * FROM person where name IS NULL;

The following syntax is incorrect, and while will not generate a syntax or runtime error, it will produce invalid results, it will return no rows: 

SELECT * FROM person where name = NULL;

TO PROCESS: https://learning.oreilly.com/library/view/learning-sql-3rd/9781492057604/ch04.html#:-:text=Null%3A%20That%20Four-Letter%20Word

Also see:

NULL

SQL Conditional Logic

SQL Conditional Logic
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