SQL SELECT: Difference between revisions

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=Clauses=
=Clauses=
==<tt>SELECT</tt>==
==<tt>SELECT</tt>==
<font>
SELECT <font color=teal>[one or more things]</font> ...
</font>
<code>SELECT</code> specifies which columns from the <code>FROM</code> sources need to be retrieved.
==<tt>FROM</tt>==
==<tt>FROM</tt>==



Revision as of 23:23, 22 May 2024

Internal

Overview

A query consists in at least one (SELECT) and at most six categories of clauses:

SELECT [one or more things] 
FROM [one or more places] 
WHERE [one or more conditions apply] 
GROUP BY [...] HAVING [...]
ORDER BY [...];

Almost every query will include at least three of these clauses (SELECT, FROM and WHERE).

Upon execution, an SQL query returns a result set.


Select all (possibly across multiple tables) then filter and discard with where.

Example

SELECT id, name FROM person WHERE id = 1;

The following query:

SELECT;

is valid, it returns one empty row.

Clauses

SELECT

SELECT [one or more things] ...

SELECT specifies which columns from the FROM sources need to be retrieved.

FROM

FROM [one or more places]

The "places" we select from can be:

  • permanent
  • derived
  • temporary
  • virtual(view)

Table alias.

Querying Multiple Tables

Querying Multiple Tables

WHERE

The WHERE Clause

GROUP BY ... HAVING

GROUP BY ... HAVING

ORDER BY

Subqueries