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Comment:Clarification of the role of ORDER BY and LIMIT in compound SELECT statements.
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SHA1: 7b8d43a8ef2404ddcc2fa5ee79873e8b3f39af4e
User & Date: drh 2014-01-30 11:08:58
Context
2014-01-30
11:15
Additional hyperlinks to "compound SELECT", "ORDER BY" and "LIMIT". check-in: 1c8ccd6d21 user: drh tags: trunk
11:08
Clarification of the role of ORDER BY and LIMIT in compound SELECT statements. check-in: 7b8d43a8ef user: drh tags: trunk
2014-01-29
17:47
Update News and the change log. Bring the release date forward to 2014-02-03. Fix typos. Move older news entries out to oldnews.in. check-in: ea6e1d11de user: drh tags: trunk
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<p>^(When three or more simple SELECTs are connected into a compound SELECT,
they group from left to right. In other words, if "A", "B" and "C" are all
simple SELECT statements, (A op B op C) is processed as ((A op B) op C).)^

</p>

<tcl>hd_fragment orderby {order by}</tcl>
<h3>ORDER BY and LIMIT/OFFSET Clauses</h3>

<p>If a SELECT statement that returns more than one row does not have an
ORDER BY clause, the order in which the rows are returned is undefined.
Or, if a SELECT statement does have an ORDER BY clause, then the list of
expressions attached to the ORDER BY determine the order in which rows






are returned to the user. ^Rows are first sorted based on the results of
evaluating the left-most expression in the ORDER BY list, then ties are broken
by evaluating the second left-most expression and so on. The order in which
two rows for which all ORDER BY expressions evaluate to equal values are
returned is undefined. ^Each ORDER BY expression may be optionally followed
by one of the keywords ASC (smaller values are returned first) or DESC (larger
values are returned first). ^If neither ASC or DESC are specified, rows
are sorted in ascending (smaller values first) order by default.
................................................................................
the second or third rules above. If a match is found, the search stops and
the expression is handled as an alias for the result column that it has been
matched against. Otherwise, the next SELECT to the right is tried, and so on.)^
^If no matching expression can be found in the result columns of any
constituent SELECT, it is an error. ^Each term of the ORDER BY clause is
processed separately and may be matched against result columns from different
SELECT statements in the compound.</p>




<p>The LIMIT clause is used to place an upper bound on the number of rows




returned by a SELECT statement. ^Any scalar expression may be used in the 
LIMIT clause, so long as it evaluates to an integer or a value that can be
losslessly converted to an integer. ^If the expression evaluates to a NULL 
value or any other value that cannot be losslessly converted to an integer, an
error is returned. ^If the LIMIT expression evaluates to a negative value,
then there is no upper bound on the number of rows returned. ^Otherwise, the
SELECT returns the first N rows of its result set only, where N is the value
that the LIMIT expression evaluates to. ^Or, if the SELECT statement would







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<p>^(When three or more simple SELECTs are connected into a compound SELECT,
they group from left to right. In other words, if "A", "B" and "C" are all
simple SELECT statements, (A op B op C) is processed as ((A op B) op C).)^

</p>

<tcl>hd_fragment orderby {order by}</tcl>
<h3>ORDER BY clauses</h3>

<p>If a SELECT statement that returns more than one row does not have an
ORDER BY clause, the order in which the rows are returned is undefined.
Or, if a SELECT statement does have an ORDER BY clause, then the list of
expressions attached to the ORDER BY determine the order in which rows
are returned to the user.
In a compound SELECT statement, only the last (right-most) SELECT may have
an ORDER BY clause and that ORDER BY clause will apply across all elements of
the compound.


<p>^Rows are first sorted based on the results of
evaluating the left-most expression in the ORDER BY list, then ties are broken
by evaluating the second left-most expression and so on. The order in which
two rows for which all ORDER BY expressions evaluate to equal values are
returned is undefined. ^Each ORDER BY expression may be optionally followed
by one of the keywords ASC (smaller values are returned first) or DESC (larger
values are returned first). ^If neither ASC or DESC are specified, rows
are sorted in ascending (smaller values first) order by default.
................................................................................
the second or third rules above. If a match is found, the search stops and
the expression is handled as an alias for the result column that it has been
matched against. Otherwise, the next SELECT to the right is tried, and so on.)^
^If no matching expression can be found in the result columns of any
constituent SELECT, it is an error. ^Each term of the ORDER BY clause is
processed separately and may be matched against result columns from different
SELECT statements in the compound.</p>

<tcl>hd_fragment limitoffset {LIMIT}</tcl>
<h3>LIMIT and OFFSET clauses</h3>

<p>The LIMIT clause is used to place an upper bound on the number of rows
returned by the entire SELECT statement.  In a compound SELECT, only the
last (right-most) SELECT may contain a LIMIT clause.  In a compound SELECT, 
the LIMIT clause applies to the entire compound, not just the final SELECT.

<p>^Any scalar expression may be used in the 
LIMIT clause, so long as it evaluates to an integer or a value that can be
losslessly converted to an integer. ^If the expression evaluates to a NULL 
value or any other value that cannot be losslessly converted to an integer, an
error is returned. ^If the LIMIT expression evaluates to a negative value,
then there is no upper bound on the number of rows returned. ^Otherwise, the
SELECT returns the first N rows of its result set only, where N is the value
that the LIMIT expression evaluates to. ^Or, if the SELECT statement would