例如, `SELECT table_name_alias.column_name FROM table_name table_name_alias`.
In the [CAST](sql_reference/functions/type_conversion_functions.md#type_conversion_function-cast) function, the `AS` keyword has another meaning. See the description of the function.
例如, `SELECT "table t".column_name FROM table_name AS "table t"`.
别名对于查询或子查询是全局的，您可以在查询的任何部分中为任何表达式定义别名。 例如, `SELECT (1 AS n) + 2, n`.
### 用法注意 {#notes-on-usage}
别名在子查询和子查询之间不可见。 例如，在执行查询时 `SELECT (SELECT sum(b.a) + num FROM b) - a.a AS num FROM a` ClickHouse生成异常 `Unknown identifier: num`.
别名在当前查询或子查询中是全局可见的，你可以在查询语句的任何位置对表达式定义别名
如果为结果列定义了别名 `SELECT` 子查询的子句，这些列在外部查询中可见。 例如, `SELECT n + m FROM (SELECT 1 AS n, 2 AS m)`.
别名在当前查询的子查询及不同子查询中是不可见的。例如，执行如下查询SQL: `SELECT (SELECT sum(b.a) + num FROM b) - a.a AS num FROM a` ,CH会提示异常 `Unknown identifier: num`.
小心使用与列或表名相同的别名。 让我们考虑以下示例:
如果给select子查询语句的结果列定义其别名，那么在外层可以使用该别名。例如, `SELECT n + m FROM (SELECT 1 AS n, 2 AS m)`.
注意列的别名和表的别名相同时的情形，考虑如下示例：
``` sql
CREATE TABLE t
(
...
...
@@ -172,16 +178,18 @@ Received exception from server (version 18.14.17):
Code: 184. DB::Exception: Received from localhost:9000, 127.0.0.1. DB::Exception: Aggregate function sum(b) is found inside another aggregate function in query.
An expression is a function, identifier, literal, application of an operator, expression in brackets, subquery, or asterisk. It can also contain an alias.
A list of expressions is one or more expressions separated by commas.
Functions and operators, in turn, can have expressions as arguments.
@@ -360,6 +360,7 @@ struct Settings : public SettingsCollection<Settings>
M(SettingBool, optimize_trivial_count_query, true, "Process trivial 'SELECT count() FROM table' query from metadata.", 0) \
M(SettingUInt64, mutations_sync, 0, "Wait for synchronous execution of ALTER TABLE UPDATE/DELETE queries (mutations). 0 - execute asynchronously. 1 - wait current server. 2 - wait all replicas if they exist.", 0) \
M(SettingBool, optimize_arithmetic_operations_in_aggregate_functions, true, "Move arithmetic operations out of aggregation functions", 0) \
M(SettingBool, optimize_duplicate_order_by_and_distinct, true, "Remove duplicate ORDER BY and DISTINCT if it's possible", 0) \
M(SettingBool, optimize_if_chain_to_miltiif, false, "Replace if(cond1, then1, if(cond2, ...)) chains to multiIf. Currently it's not beneficial for numeric types.", 0) \
M(SettingBool, allow_experimental_alter_materialized_view_structure, false, "Allow atomic alter on Materialized views. Work in progress.", 0) \
M(SettingBool, enable_early_constant_folding, true, "Enable query optimization where we analyze function and subqueries results and rewrite query if there're constants there", 0) \
...
...
@@ -376,6 +377,7 @@ struct Settings : public SettingsCollection<Settings>
M(SettingBool, materialize_ttl_after_modify, true, "Apply TTL for old data, after ALTER MODIFY TTL query", 0) \
\
M(SettingBool, allow_experimental_geo_types, false, "Allow geo data types such as Point, Ring, Polygon, MultiPolygon", 0) \
M(SettingBool, data_type_default_nullable, false, "Data types without NULL or NOT NULL will make Nullable", 0) \
\
/** Obsolete settings that do nothing but left for compatibility reasons. Remove each one after half a year of obsolescence. */ \
<query>SELECT * FROM (SELECT CounterID, EventDate FROM hits_10m_single ORDER BY CounterID DESC) ORDER BY EventDate, CounterID FORMAT Null</query>
<query>SELECT DISTINCT * FROM (SELECT DISTINCT CounterID, EventDate FROM hits_10m_single) FORMAT Null</query>
<query>SELECT DISTINCT * FROM (SELECT DISTINCT CounterID, EventDate FROM hits_10m_single ORDER BY CounterID DESC) ORDER BY toStartOfWeek(EventDate) FORMAT Null</query>
SELECT number\nFROM \n(\n SELECT number\n FROM \n (\n SELECT DISTINCT number\n FROM numbers(3)\n )\n)\nORDER BY number ASC
0
1
2
SELECT DISTINCT number\nFROM \n(\n SELECT DISTINCT number\n FROM \n (\n SELECT DISTINCT number\n FROM numbers(3)\n ORDER BY number ASC\n )\n ORDER BY number ASC\n)\nORDER BY number ASC
0
1
2
SELECT number\nFROM \n(\n SELECT DISTINCT number\n FROM \n (\n SELECT DISTINCT number % 2 AS number\n FROM numbers(3)\n )\n)\nORDER BY number ASC
0
1
SELECT DISTINCT number\nFROM \n(\n SELECT DISTINCT number\n FROM \n (\n SELECT DISTINCT number % 2 AS number\n FROM numbers(3)\n ORDER BY number ASC\n )\n ORDER BY number ASC\n)\nORDER BY number ASC
