Sophie: postgresql9.6-docs-9.6.22-1.mga7 noarch

postgresql9.6-docs-9.6.22-1.mga7.noarch.rpm

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<HTML
><HEAD
><TITLE
>UNION, CASE, and Related Constructs</TITLE
><META
NAME="GENERATOR"
CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
REV="MADE"
HREF="mailto:pgsql-docs@postgresql.org"><LINK
REL="HOME"
TITLE="PostgreSQL 9.6.22 Documentation"
HREF="index.html"><LINK
REL="UP"
TITLE="Type Conversion"
HREF="typeconv.html"><LINK
REL="PREVIOUS"
TITLE="Value Storage"
HREF="typeconv-query.html"><LINK
REL="NEXT"
TITLE="Indexes"
HREF="indexes.html"><LINK
REL="STYLESHEET"
TYPE="text/css"
HREF="stylesheet.css"><META
HTTP-EQUIV="Content-Type"
CONTENT="text/html; charset=ISO-8859-1"><META
NAME="creation"
CONTENT="2021-05-18T09:16:10"></HEAD
><BODY
CLASS="SECT1"
><DIV
CLASS="NAVHEADER"
><TABLE
SUMMARY="Header navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TH
COLSPAN="4"
ALIGN="center"
VALIGN="bottom"
><A
HREF="index.html"
>PostgreSQL 9.6.22 Documentation</A
></TH
></TR
><TR
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
TITLE="Value Storage"
HREF="typeconv-query.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="10%"
ALIGN="left"
VALIGN="top"
><A
HREF="typeconv.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="60%"
ALIGN="center"
VALIGN="bottom"
>Chapter 10. Type Conversion</TD
><TD
WIDTH="20%"
ALIGN="right"
VALIGN="top"
><A
TITLE="Indexes"
HREF="indexes.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="100%"></DIV
><DIV
CLASS="SECT1"
><H1
CLASS="SECT1"
><A
NAME="TYPECONV-UNION-CASE"
>10.5. <TT
CLASS="LITERAL"
>UNION</TT
>, <TT
CLASS="LITERAL"
>CASE</TT
>, and Related Constructs</A
></H1
><P
>SQL <TT
CLASS="LITERAL"
>UNION</TT
> constructs must match up possibly dissimilar
types to become a single result set.  The resolution algorithm is
applied separately to each output column of a union query.  The
<TT
CLASS="LITERAL"
>INTERSECT</TT
> and <TT
CLASS="LITERAL"
>EXCEPT</TT
> constructs resolve
dissimilar types in the same way as <TT
CLASS="LITERAL"
>UNION</TT
>.
Some other constructs, including
<TT
CLASS="LITERAL"
>CASE</TT
>, <TT
CLASS="LITERAL"
>ARRAY</TT
>, <TT
CLASS="LITERAL"
>VALUES</TT
>,
and the <CODE
CLASS="FUNCTION"
>GREATEST</CODE
> and <CODE
CLASS="FUNCTION"
>LEAST</CODE
>
functions, use the identical
algorithm to match up their component expressions and select a result
data type.</P
><DIV
CLASS="PROCEDURE"
><P
><B
>Type Resolution for <TT
CLASS="LITERAL"
>UNION</TT
>, <TT
CLASS="LITERAL"
>CASE</TT
>,
and Related Constructs</B
></P
><OL
TYPE="1"
><LI
CLASS="STEP"
><P
>If all inputs are of the same type, and it is not <TT
CLASS="TYPE"
>unknown</TT
>,
resolve as that type.</P
></LI
><LI
CLASS="STEP"
><P
>If any input is of a domain type, treat it as being of the
domain's base type for all subsequent steps.
  <A
NAME="AEN25740"
HREF="#FTN.AEN25740"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></P
></LI
><LI
CLASS="STEP"
><P
>If all inputs are of type <TT
CLASS="TYPE"
>unknown</TT
>, resolve as type
<TT
CLASS="TYPE"
>text</TT
> (the preferred type of the string category).
Otherwise, <TT
CLASS="TYPE"
>unknown</TT
> inputs are ignored.</P
></LI
><LI
CLASS="STEP"
><P
>If the non-unknown inputs are not all of the same type category, fail.</P
></LI
><LI
CLASS="STEP"
><P
>Select the first non-unknown input type as the candidate type,
then consider each other non-unknown input type, left to right.
  <A
NAME="AEN25752"
HREF="#FTN.AEN25752"
><SPAN
CLASS="footnote"
>[2]</SPAN
></A
>
If the candidate type can be implicitly converted to the other type,
but not vice-versa, select the other type as the new candidate type.
Then continue considering the remaining inputs.  If, at any stage of this
process, a preferred type is selected, stop considering additional
inputs.</P
></LI
><LI
CLASS="STEP"
><P
>Convert all inputs to the final candidate type.  Fail if there is not an
implicit conversion from a given input type to the candidate type.</P
></LI
></OL
></DIV
><P
>Some examples follow.</P
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN25762"
></A
><P
><B
>Example 10-10. Type Resolution with Underspecified Types in a Union</B
></P
><P
></P><PRE
CLASS="SCREEN"
>SELECT text 'a' AS "text" UNION SELECT 'b';

 text
------
 a
 b
(2 rows)</PRE
><P>
Here, the unknown-type literal <TT
CLASS="LITERAL"
>'b'</TT
> will be resolved to type <TT
CLASS="TYPE"
>text</TT
>.</P
></DIV
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN25768"
></A
><P
><B
>Example 10-11. Type Resolution in a Simple Union</B
></P
><P
></P><PRE
CLASS="SCREEN"
>SELECT 1.2 AS "numeric" UNION SELECT 1;

 numeric
---------
       1
     1.2
(2 rows)</PRE
><P>
The literal <TT
CLASS="LITERAL"
>1.2</TT
> is of type <TT
CLASS="TYPE"
>numeric</TT
>,
and the <TT
CLASS="TYPE"
>integer</TT
> value <TT
CLASS="LITERAL"
>1</TT
> can be cast implicitly to
<TT
CLASS="TYPE"
>numeric</TT
>, so that type is used.</P
></DIV
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN25777"
></A
><P
><B
>Example 10-12. Type Resolution in a Transposed Union</B
></P
><P
></P><PRE
CLASS="SCREEN"
>SELECT 1 AS "real" UNION SELECT CAST('2.2' AS REAL);

 real
------
    1
  2.2
(2 rows)</PRE
><P>
Here, since type <TT
CLASS="TYPE"
>real</TT
> cannot be implicitly cast to <TT
CLASS="TYPE"
>integer</TT
>,
but <TT
CLASS="TYPE"
>integer</TT
> can be implicitly cast to <TT
CLASS="TYPE"
>real</TT
>, the union
result type is resolved as <TT
CLASS="TYPE"
>real</TT
>.</P
></DIV
><DIV
CLASS="EXAMPLE"
><A
NAME="AEN25786"
></A
><P
><B
>Example 10-13. Type Resolution in a Nested Union</B
></P
><P
></P><PRE
CLASS="SCREEN"
>SELECT NULL UNION SELECT NULL UNION SELECT 1;

ERROR:  UNION types text and integer cannot be matched</PRE
><P>
This failure occurs because <SPAN
CLASS="PRODUCTNAME"
>PostgreSQL</SPAN
> treats
multiple <TT
CLASS="LITERAL"
>UNION</TT
>s as a nest of pairwise operations;
that is, this input is the same as
</P><PRE
CLASS="SCREEN"
>(SELECT NULL UNION SELECT NULL) UNION SELECT 1;</PRE
><P>
The inner <TT
CLASS="LITERAL"
>UNION</TT
> is resolved as emitting
type <TT
CLASS="TYPE"
>text</TT
>, according to the rules given above.  Then the
outer <TT
CLASS="LITERAL"
>UNION</TT
> has inputs of types <TT
CLASS="TYPE"
>text</TT
>
and <TT
CLASS="TYPE"
>integer</TT
>, leading to the observed error.  The problem
can be fixed by ensuring that the leftmost <TT
CLASS="LITERAL"
>UNION</TT
>
has at least one input of the desired result type.</P
><P
><TT
CLASS="LITERAL"
>INTERSECT</TT
> and <TT
CLASS="LITERAL"
>EXCEPT</TT
> operations are
likewise resolved pairwise.  However, the other constructs described in this
section consider all of their inputs in one resolution step.</P
></DIV
></DIV
><H3
CLASS="FOOTNOTES"
>Notes</H3
><TABLE
BORDER="0"
CLASS="FOOTNOTES"
WIDTH="100%"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN25740"
HREF="typeconv-union-case.html#AEN25740"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>    Somewhat like the treatment of domain inputs for operators and
    functions, this behavior allows a domain type to be preserved through
    a <TT
CLASS="LITERAL"
>UNION</TT
> or similar construct, so long as the user is
    careful to ensure that all inputs are implicitly or explicitly of that
    exact type.  Otherwise the domain's base type will be used.
   </P
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="5%"
><A
NAME="FTN.AEN25752"
HREF="typeconv-union-case.html#AEN25752"
><SPAN
CLASS="footnote"
>[2]</SPAN
></A
></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="95%"
><P
>    For historical reasons, <TT
CLASS="LITERAL"
>CASE</TT
> treats
    its <TT
CLASS="LITERAL"
>ELSE</TT
> clause (if any) as the <SPAN
CLASS="QUOTE"
>"first"</SPAN
>
    input, with the <TT
CLASS="LITERAL"
>THEN</TT
> clauses(s) considered after
    that.  In all other cases, <SPAN
CLASS="QUOTE"
>"left to right"</SPAN
> means the order
    in which the expressions appear in the query text.
   </P
></TD
></TR
></TABLE
><DIV
CLASS="NAVFOOTER"
><HR
ALIGN="LEFT"
WIDTH="100%"><TABLE
SUMMARY="Footer navigation table"
WIDTH="100%"
BORDER="0"
CELLPADDING="0"
CELLSPACING="0"
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
><A
HREF="typeconv-query.html"
ACCESSKEY="P"
>Prev</A
></TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="index.html"
ACCESSKEY="H"
>Home</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
><A
HREF="indexes.html"
ACCESSKEY="N"
>Next</A
></TD
></TR
><TR
><TD
WIDTH="33%"
ALIGN="left"
VALIGN="top"
>Value Storage</TD
><TD
WIDTH="34%"
ALIGN="center"
VALIGN="top"
><A
HREF="typeconv.html"
ACCESSKEY="U"
>Up</A
></TD
><TD
WIDTH="33%"
ALIGN="right"
VALIGN="top"
>Indexes</TD
></TR
></TABLE
></DIV
></BODY
></HTML
>