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impala/docs/topics/impala_conversion_functions.xml
John Russell 8377b9949c Global search/replace: audience="Cloudera" -> audience="hidden". 
For this change to land in master, the audience="hidden" code review
needs to be completed first. Otherwise, the doc build would still work
but the audience="hidden" content would be visible rather than hidden as
desired.

Some work happening in parallel might introduce additional instances of
audience="Cloudera". I suggest addressing those in a followup CR so this
global change can land quickly.

Since the changes apply across so many different files, but are so
narrow in scope, I suggest that the way to validate (check that no
extraneous changes were introduced accidentally) is to diff just the
changed lines:

git diff -U0 HEAD^ HEAD

In patch set 2, I updated other topics marked audience="Cloudera"
by CRs that were pushed in the meantime.

Change-Id: Ic93d89da77e1f51bbf548a522d98d0c4e2fb31c8
Reviewed-on: http://gerrit.cloudera.org:8080/5613
Reviewed-by: John Russell <jrussell@cloudera.com>
Tested-by: Impala Public Jenkins
2017-01-18 19:31:57 +00:00

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<?xml version="1.0" encoding="UTF-8"?><!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements. See the NOTICE file
distributed with this work for additional information
regarding copyright ownership. The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License. You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied. See the License for the
specific language governing permissions and limitations
under the License.
-->
<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="conversion_functions">
<title>Impala Type Conversion Functions</title>
<titlealts audience="PDF"><navtitle>Type Conversion Functions</navtitle></titlealts>
<prolog>
<metadata>
<data name="Category" value="Impala"/>
<data name="Category" value="Impala Functions"/>
<data name="Category" value="Impala Data Types"/>
<data name="Category" value="SQL"/>
<data name="Category" value="Data Analysts"/>
<data name="Category" value="Developers"/>
<data name="Category" value="Querying"/>
</metadata>
</prolog>
<conbody>
<p>
Conversion functions are usually used in combination with other functions, to explicitly pass the expected
data types. Impala has strict rules regarding data types for function parameters. For example, Impala does
not automatically convert a <codeph>DOUBLE</codeph> value to <codeph>FLOAT</codeph>, a
<codeph>BIGINT</codeph> value to <codeph>INT</codeph>, or other conversion where precision could be lost or
overflow could occur. Also, for reporting or dealing with loosely defined schemas in big data contexts,
you might frequently need to convert values to or from the <codeph>STRING</codeph> type.
</p>
<note>
Although in <keyword keyref="impala23_full"/>, the <codeph>SHOW FUNCTIONS</codeph> output for
database <codeph>_IMPALA_BUILTINS</codeph> contains some function signatures
matching the pattern <codeph>castto*</codeph>, these functions are not intended
for public use and are expected to be hidden in future.
</note>
<p>
<b>Function reference:</b>
</p>
<p>
Impala supports the following type conversion functions:
</p>
<dl>
<dlentry id="cast">
<dt>
<codeph>cast(<varname>expr</varname> AS <varname>type</varname>)</codeph>
</dt>
<dd>
<indexterm audience="hidden">cast() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to any other type.
If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Usage notes:</b>
Use <codeph>CAST</codeph> when passing a column value or literal to a function that
expects a parameter with a different type.
Frequently used in SQL operations such as <codeph>CREATE TABLE AS SELECT</codeph>
and <codeph>INSERT ... VALUES</codeph> to ensure that values from various sources
are of the appropriate type for the destination columns.
Where practical, do a one-time <codeph>CAST()</codeph> operation during the ingestion process
to make each column into the appropriate type, rather than using many <codeph>CAST()</codeph>
operations in each query; doing type conversions for each row during each query can be expensive
for tables with millions or billions of rows.
</p>
<p conref="../shared/impala_common.xml#common/timezone_conversion_caveat"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<codeblock>select concat('Here are the first ',10,' results.'); -- Fails
select concat('Here are the first ',cast(10 as string),' results.'); -- Succeeds
</codeblock>
<p>
The following example starts with a text table where every column has a type of <codeph>STRING</codeph>,
which might be how you ingest data of unknown schema until you can verify the cleanliness of the underly values.
Then it uses <codeph>CAST()</codeph> to create a new Parquet table with the same data, but using specific
numeric data types for the columns with numeric data. Using numeric types of appropriate sizes can result in
substantial space savings on disk and in memory, and performance improvements in queries,
over using strings or larger-than-necessary numeric types.
</p>
<codeblock>create table t1 (name string, x string, y string, z string);
create table t2 stored as parquet
as select
name,
cast(x as bigint) x,
cast(y as timestamp) y,
cast(z as smallint) z
from t1;
describe t2;
+------+----------+---------+
| name | type | comment |
+------+----------+---------+
| name | string | |
| x | bigint | |
| y | smallint | |
| z | tinyint | |
+------+----------+---------+
</codeblock>
<p conref="../shared/impala_common.xml#common/related_info"/>
<p>
<!-- TK: Can you cast to or from MAP, ARRAY, STRUCT? -->
For details of casts from each kind of data type, see the description of
the appropriate type:
<xref href="impala_tinyint.xml#tinyint"/>,
<xref href="impala_smallint.xml#smallint"/>,
<xref href="impala_int.xml#int"/>,
<xref href="impala_bigint.xml#bigint"/>,
<xref href="impala_float.xml#float"/>,
<xref href="impala_double.xml#double"/>,
<xref href="impala_decimal.xml#decimal"/>,
<xref href="impala_string.xml#string"/>,
<xref href="impala_char.xml#char"/>,
<xref href="impala_varchar.xml#varchar"/>,
<xref href="impala_timestamp.xml#timestamp"/>,
<xref href="impala_boolean.xml#boolean"/>
</p>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttobigint" audience="hidden">
<dt>
<codeph>casttobigint(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttobigint() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>BIGINT</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>bigint</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>create table small_types (x tinyint, y smallint, z int);
create table big_types as
select casttobigint(x) as x, casttobigint(y) as y, casttobigint(z) as z
from small_types;
describe big_types;
+------+--------+---------+
| name | type | comment |
+------+--------+---------+
| x | bigint | |
| y | bigint | |
| z | bigint | |
+------+--------+---------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttoboolean" audience="hidden">
<dt>
<codeph>casttoboolean(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttoboolean() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>BOOLEAN</codeph>.
Numeric values of 0 evaluate to <codeph>false</codeph>, and non-zero values evaluate to <codeph>true</codeph>.
If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
In particular, <codeph>STRING</codeph> values (even <codeph>'1'</codeph>, <codeph>'0'</codeph>, <codeph>'true'</codeph>
or <codeph>'false'</codeph>) always return <codeph>NULL</codeph> when converted to <codeph>BOOLEAN</codeph>.
<p><b>Return type:</b> <codeph>boolean</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>select casttoboolean(0);
+------------------+
| casttoboolean(0) |
+------------------+
| false |
+------------------+
select casttoboolean(1);
+------------------+
| casttoboolean(1) |
+------------------+
| true |
+------------------+
select casttoboolean(99);
+-------------------+
| casttoboolean(99) |
+-------------------+
| true |
+-------------------+
select casttoboolean(0.0);
+--------------------+
| casttoboolean(0.0) |
+--------------------+
| false |
+--------------------+
select casttoboolean(0.5);
+--------------------+
| casttoboolean(0.5) |
+--------------------+
| true |
+--------------------+
select casttoboolean('');
+-------------------+
| casttoboolean('') |
+-------------------+
| NULL |
+-------------------+
select casttoboolean('yes');
+----------------------+
| casttoboolean('yes') |
+----------------------+
| NULL |
+----------------------+
select casttoboolean('0');
+--------------------+
| casttoboolean('0') |
+--------------------+
| NULL |
+--------------------+
select casttoboolean('true');
+-----------------------+
| casttoboolean('true') |
+-----------------------+
| NULL |
+-----------------------+
select casttoboolean('false');
+------------------------+
| casttoboolean('false') |
+------------------------+
| NULL |
+------------------------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttochar" audience="hidden">
<dt>
<codeph>casttochar(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttochar() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>CHAR</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>char</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>create table char_types as select casttochar('hello world') as c1, casttochar('xyz') as c2, casttochar('x') as c3;
+-------------------+
| summary |
+-------------------+
| Inserted 1 row(s) |
+-------------------+
describe char_types;
+------+--------+---------+
| name | type | comment |
+------+--------+---------+
| c1 | string | |
| c2 | string | |
| c3 | string | |
+------+--------+---------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttodecimal" audience="hidden">
<dt>
<codeph>casttodecimal(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttodecimal() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>DECIMAL</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>decimal</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>select casttodecimal(5.4);
+--------------------+
| casttodecimal(5.4) |
+--------------------+
| 5.4 |
+--------------------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttodouble" audience="hidden">
<dt>
<codeph>casttodouble(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttodouble() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>DOUBLE</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>double</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>select casttodouble(5);
+-----------------+
| casttodouble(5) |
+-----------------+
| 5 |
+-----------------+
select casttodouble('3.141');
+-----------------------+
| casttodouble('3.141') |
+-----------------------+
| 3.141 |
+-----------------------+
select casttodouble(1e6);
+--------------------+
| casttodouble(1e+6) |
+--------------------+
| 1000000 |
+--------------------+
select casttodouble(true);
+--------------------+
| casttodouble(true) |
+--------------------+
| 1 |
+--------------------+
select casttodouble(now());
+---------------------+
| casttodouble(now()) |
+---------------------+
| 1447622306.031178 |
+---------------------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttofloat" audience="hidden">
<dt>
<codeph>casttofloat(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttofloat() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>FLOAT</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>float</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>select casttofloat(5);
+----------------+
| casttofloat(5) |
+----------------+
| 5 |
+----------------+
select casttofloat('3.141');
+----------------------+
| casttofloat('3.141') |
+----------------------+
| 3.141000032424927 |
+----------------------+
select casttofloat(1e6);
+-------------------+
| casttofloat(1e+6) |
+-------------------+
| 1000000 |
+-------------------+
select casttofloat(true);
+-------------------+
| casttofloat(true) |
+-------------------+
| 1 |
+-------------------+
select casttofloat(now());
+--------------------+
| casttofloat(now()) |
+--------------------+
| 1447622400 |
+--------------------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttoint" audience="hidden">
<dt>
<codeph>casttoint(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttoint() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>INT</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>int</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>select casttoint(5.4);
+----------------+
| casttoint(5.4) |
+----------------+
| 5 |
+----------------+
select casttoint(true);
+-----------------+
| casttoint(true) |
+-----------------+
| 1 |
+-----------------+
select casttoint(now());
+------------------+
| casttoint(now()) |
+------------------+
| 1447622487 |
+------------------+
select casttoint('3.141');
+--------------------+
| casttoint('3.141') |
+--------------------+
| NULL |
+--------------------+
select casttoint('3');
+----------------+
| casttoint('3') |
+----------------+
| 3 |
+----------------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttosmallint" audience="hidden">
<dt>
<codeph>casttosmallint(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttosmallint() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>SMALLINT</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>smallint</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>create table big_types (x bigint, y int, z smallint);
create table small_types as
select casttosmallint(x) as x, casttosmallint(y) as y, casttosmallint(z) as z
from big_types;
describe small_types;
+------+----------+---------+
| name | type | comment |
+------+----------+---------+
| x | smallint | |
| y | smallint | |
| z | smallint | |
+------+----------+---------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttostring" audience="hidden">
<dt>
<codeph>casttostring(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttostring() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>STRING</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>string</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>create table numeric_types (x int, y bigint, z tinyint);
create table string_types as
select casttostring(x) as x, casttostring(y) as y, casttostring(z) as z
from numeric_types;
describe string_types;
+------+--------+---------+
| name | type | comment |
+------+--------+---------+
| x | string | |
| y | string | |
| z | string | |
+------+--------+---------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttotimestamp" audience="hidden">
<dt>
<codeph>casttotimestamp(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttotimestamp() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>TIMESTAMP</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>timestamp</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>select casttotimestamp(1000);
+-----------------------+
| casttotimestamp(1000) |
+-----------------------+
| 1970-01-01 00:16:40 |
+-----------------------+
select casttotimestamp(1000.0);
+-------------------------+
| casttotimestamp(1000.0) |
+-------------------------+
| 1970-01-01 00:16:40 |
+-------------------------+
select casttotimestamp('1000');
+-------------------------+
| casttotimestamp('1000') |
+-------------------------+
| NULL |
+-------------------------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttotinyint" audience="hidden">
<dt>
<codeph>casttotinyint(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttotinyint() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>TINYINT</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>tinyint</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>create table big_types (x bigint, y int, z smallint);
create table tiny_types as
select casttotinyint(x) as x, casttotinyint(y) as y, casttotinyint(z) as z
from big_types;
describe tiny_types;
+------+---------+---------+
| name | type | comment |
+------+---------+---------+
| x | tinyint | |
| y | tinyint | |
| z | tinyint | |
+------+---------+---------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="casttovarchar" audience="hidden">
<dt>
<codeph>casttovarchar(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">casttovarchar() function</indexterm>
<b>Purpose:</b> Converts the value of an expression to <codeph>VARCHAR</codeph>. If the expression value is of a type that cannot be converted to the target type, the result is <codeph>NULL</codeph>.
<p><b>Return type:</b> <codeph>varchar</codeph></p>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_usage"/>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p conref="../shared/impala_common.xml#common/cast_convenience_fn_example"/>
<codeblock>select casttovarchar('abcd');
+-----------------------+
| casttovarchar('abcd') |
+-----------------------+
| abcd |
+-----------------------+
select casttovarchar(999);
+--------------------+
| casttovarchar(999) |
+--------------------+
| 999 |
+--------------------+
select casttovarchar(999.5);
+----------------------+
| casttovarchar(999.5) |
+----------------------+
| 999.5 |
+----------------------+
select casttovarchar(now());
+-------------------------------+
| casttovarchar(now()) |
+-------------------------------+
| 2015-11-15 21:26:13.528073000 |
+-------------------------------+
select casttovarchar(true);
+---------------------+
| casttovarchar(true) |
+---------------------+
| 1 |
+---------------------+
</codeblock>
</dd>
</dlentry>
<dlentry rev="2.3.0" id="typeof">
<dt>
<codeph>typeof(type value)</codeph>
</dt>
<dd>
<indexterm audience="hidden">typeof() function</indexterm>
<b>Purpose:</b> Returns the name of the data type corresponding to an expression. For types with
extra attributes, such as length for <codeph>CHAR</codeph> and <codeph>VARCHAR</codeph>,
or precision and scale for <codeph>DECIMAL</codeph>, includes the full specification of the type.
<!-- To do: How about for columns of complex types? Or fields within complex types? -->
<p><b>Return type:</b> <codeph>string</codeph></p>
<p><b>Usage notes:</b> Typically used in interactive exploration of a schema, or in application code that programmatically generates schema definitions such as <codeph>CREATE TABLE</codeph> statements.
For example, previously, to understand the type of an expression such as
<codeph>col1 / col2</codeph> or <codeph>concat(col1, col2, col3)</codeph>,
you might have created a dummy table with a single row, using syntax such as <codeph>CREATE TABLE foo AS SELECT 5 / 3.0</codeph>,
and then doing a <codeph>DESCRIBE</codeph> to see the type of the row.
Or you might have done a <codeph>CREATE TABLE AS SELECT</codeph> operation to create a table and
copy data into it, only learning the types of the columns by doing a <codeph>DESCRIBE</codeph> afterward.
This technique is especially useful for arithmetic expressions involving <codeph>DECIMAL</codeph> types,
because the precision and scale of the result is typically different than that of the operands.
</p>
<p conref="../shared/impala_common.xml#common/added_in_230"/>
<p conref="../shared/impala_common.xml#common/example_blurb"/>
<p>
These examples show how to check the type of a simple literal or function value.
Notice how adding even tiny integers together changes the data type of the result to
avoid overflow, and how the results of arithmetic operations on <codeph>DECIMAL</codeph> values
have specific precision and scale attributes.
</p>
<codeblock>select typeof(2)
+-----------+
| typeof(2) |
+-----------+
| TINYINT |
+-----------+
select typeof(2+2)
+---------------+
| typeof(2 + 2) |
+---------------+
| SMALLINT |
+---------------+
select typeof('xyz')
+---------------+
| typeof('xyz') |
+---------------+
| STRING |
+---------------+
select typeof(now())
+---------------+
| typeof(now()) |
+---------------+
| TIMESTAMP |
+---------------+
select typeof(5.3 / 2.1)
+-------------------+
| typeof(5.3 / 2.1) |
+-------------------+
| DECIMAL(6,4) |
+-------------------+
select typeof(5.30001 / 2342.1);
+--------------------------+
| typeof(5.30001 / 2342.1) |
+--------------------------+
| DECIMAL(13,11) |
+--------------------------+
select typeof(typeof(2+2))
+-----------------------+
| typeof(typeof(2 + 2)) |
+-----------------------+
| STRING |
+-----------------------+
</codeblock>
<p>
This example shows how even if you do not have a record of the type of a column,
for example because the type was changed by <codeph>ALTER TABLE</codeph> after the
original <codeph>CREATE TABLE</codeph>, you can still find out the type in a
more compact form than examining the full <codeph>DESCRIBE</codeph> output.
Remember to use <codeph>LIMIT 1</codeph> in such cases, to avoid an identical
result value for every row in the table.
</p>
<codeblock>create table typeof_example (a int, b tinyint, c smallint, d bigint);
/* Empty result set if there is no data in the table. */
select typeof(a) from typeof_example;
/* OK, now we have some data but the type of column A is being changed. */
insert into typeof_example values (1, 2, 3, 4);
alter table typeof_example change a a bigint;
/* We can always find out the current type of that column without doing a full DESCRIBE. */
select typeof(a) from typeof_example limit 1;
+-----------+
| typeof(a) |
+-----------+
| BIGINT |
+-----------+
</codeblock>
<p>
This example shows how you might programmatically generate a <codeph>CREATE TABLE</codeph> statement
with the appropriate column definitions to hold the result values of arbitrary expressions.
The <codeph>typeof()</codeph> function lets you construct a detailed <codeph>CREATE TABLE</codeph> statement
without actually creating the table, as opposed to <codeph>CREATE TABLE AS SELECT</codeph> operations
where you create the destination table but only learn the column data types afterward through <codeph>DESCRIBE</codeph>.
</p>
<codeblock>describe typeof_example;
+------+----------+---------+
| name | type | comment |
+------+----------+---------+
| a | bigint | |
| b | tinyint | |
| c | smallint | |
| d | bigint | |
+------+----------+---------+
/* An ETL or business intelligence tool might create variations on a table with different file formats,
different sets of columns, and so on. TYPEOF() lets an application introspect the types of the original columns. */
select concat('create table derived_table (a ', typeof(a), ', b ', typeof(b), ', c ',
typeof(c), ', d ', typeof(d), ') stored as parquet;')
as 'create table statement'
from typeof_example limit 1;
+-------------------------------------------------------------------------------------------+
| create table statement |
+-------------------------------------------------------------------------------------------+
| create table derived_table (a BIGINT, b TINYINT, c SMALLINT, d BIGINT) stored as parquet; |
+-------------------------------------------------------------------------------------------+
</codeblock>
</dd>
</dlentry>
</dl>
</conbody>
</concept>
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