# DESCRIBE
为指定表显示一个或多个列，包括分区列。此命令对于检查复杂列的属性非常有用。
## 摘要
```
DESCRIBE [EXTENDED | FORMATTED] [db_name.]table_name [PARTITION partition_spec] [col_name ( [.field_name] | [.'$elem$'] | [.'$key$'] | [.'$value$'] )]
```
**重要**
此语句的语法为 `DESCRIBE {{table_name}}` 而非 `DESCRIBE TABLE {{table_name}}`。使用后一种语法会导致出现错误消息 FAILED: SemanticException [Error 10001]: Table not found table（失败：SemanticException [错误 10001]：找不到表）。
## 参数
**[EXTENDED \| FORMATTED]**
确定输出的格式。省略这些参数会以表格格式显示列名称及其对应的数据类型，包括分区列。指定 `FORMATTED` 不仅会以表格格式显示列名和数据类型，还会显示详细的表格和存储信息。`EXTENDED` 会以表格格式显示列和数据类型信息，并以 Thrift 序列化形式显示表的详细元数据。此格式的可读性较差，因而主要用于调试。
**[PARTITION partition\_spec]**
如果包括在内，则列出由 `partition_spec` 指定的分区的元数据，其中 `partition_spec` 的格式为 `(partition_column = partition_col_value, partition_column = partition_col_value, ...)`。
**[col\_name ( [.field\_name] \| [.'$elem$'] \| [.'$key$'] \| [.'$value$'] )\* ]**
指定要检查的列和属性。您可以为结构的元素指定 `.field_name`，为数组元素指定 `'$elem$'`，为映射键指定 `'$key$'`，为映射值指定 `'$value$'`。您可以以递归方式指定它，以便进一步探索复杂的列。
### 示例
```
DESCRIBE orders
```
```
DESCRIBE FORMATTED mydatabase.mytable PARTITION (part_col = 100) columnA;
```
下面的查询和输出显示了来自 `impressions` 表的列和数据类型信息，该表基于 Amazon EMR 示例数据。
```
DESCRIBE impressions
```
```
requestbegintime string from deserializer
adid string from deserializer
impressionid string from deserializer
referrer string from deserializer
useragent string from deserializer
usercookie string from deserializer
ip string from deserializer
number string from deserializer
processid string from deserializer
browsercokie string from deserializer
requestendtime string from deserializer
timers struct from deserializer
threadid string from deserializer
hostname string from deserializer
sessionid string from deserializer
dt string
# Partition Information
# col_name data_type comment
dt string
```
下面的示例查询和输出显示了使用 `FORMATTED` 选项时相同表的结果。
```
DESCRIBE FORMATTED impressions
```
```
requestbegintime string from deserializer
adid string from deserializer
impressionid string from deserializer
referrer string from deserializer
useragent string from deserializer
usercookie string from deserializer
ip string from deserializer
number string from deserializer
processid string from deserializer
browsercokie string from deserializer
requestendtime string from deserializer
timers struct from deserializer
threadid string from deserializer
hostname string from deserializer
sessionid string from deserializer
dt string
# Partition Information
# col_name data_type comment
dt string
# Detailed Table Information
Database: sampledb
Owner: hadoop
CreateTime: Thu Apr 23 02:55:21 UTC 2020
LastAccessTime: UNKNOWN
Protect Mode: None
Retention: 0
Location: s3://us-east-1.elasticmapreduce/samples/hive-ads/tables/impressions
Table Type: EXTERNAL_TABLE
Table Parameters:
EXTERNAL TRUE
transient_lastDdlTime 1587610521
# Storage Information
SerDe Library: org.openx.data.jsonserde.JsonSerDe
InputFormat: org.apache.hadoop.mapred.TextInputFormat
OutputFormat: org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat
Compressed: No
Num Buckets: -1
Bucket Columns: []
Sort Columns: []
Storage Desc Params:
paths requestbegintime, adid, impressionid, referrer, useragent, usercookie, ip
serialization.format 1
```
下面的示例查询和输出显示了使用 `EXTENDED` 选项时相同表的结果。表的详细信息会输出到一行中，但是为了便于阅读，此处设置了格式。
```
DESCRIBE EXTENDED impressions
```
```
requestbegintime string from deserializer
adid string from deserializer
impressionid string from deserializer
referrer string from deserializer
useragent string from deserializer
usercookie string from deserializer
ip string from deserializer
number string from deserializer
processid string from deserializer
browsercokie string from deserializer
requestendtime string from deserializer
timers struct from deserializer
threadid string from deserializer
hostname string from deserializer
sessionid string from deserializer
dt string
# Partition Information
# col_name data_type comment
dt string
Detailed Table Information Table(tableName:impressions, dbName:sampledb, owner:hadoop, createTime:1587610521,
lastAccessTime:0, retention:0, sd:StorageDescriptor(cols:[FieldSchema(name:requestbegintime, type:string, comment:null),
FieldSchema(name:adid, type:string, comment:null), FieldSchema(name:impressionid, type:string, comment:null),
FieldSchema(name:referrer, type:string, comment:null), FieldSchema(name:useragent, type:string, comment:null),
FieldSchema(name:usercookie, type:string, comment:null), FieldSchema(name:ip, type:string, comment:null),
FieldSchema(name:number, type:string, comment:null), FieldSchema(name:processid, type:string, comment:null),
FieldSchema(name:browsercokie, type:string, comment:null), FieldSchema(name:requestendtime, type:string, comment:null),
FieldSchema(name:timers, type:struct, comment:null), FieldSchema(name:threadid,
type:string, comment:null), FieldSchema(name:hostname, type:string, comment:null), FieldSchema(name:sessionid,
type:string, comment:null)], location:s3://us-east-1.elasticmapreduce/samples/hive-ads/tables/impressions,
inputFormat:org.apache.hadoop.mapred.TextInputFormat,
outputFormat:org.apache.hadoop.hive.ql.io.IgnoreKeyTextOutputFormat, compressed:false, numBuckets:-1,
serdeInfo:SerDeInfo(name:null, serializationLib:org.openx.data.jsonserde.JsonSerDe, parameters:{serialization.format=1,
paths=requestbegintime, adid, impressionid, referrer, useragent, usercookie, ip}), bucketCols:[], sortCols:[], parameters:{},
skewedInfo:SkewedInfo(skewedColNames:[], skewedColValues:[], skewedColValueLocationMaps:{}),
storedAsSubDirectories:false), partitionKeys:[FieldSchema(name:dt, type:string, comment:null)],
parameters:{EXTERNAL=TRUE, transient_lastDdlTime=1587610521}, viewOriginalText:null, viewExpandedText:null,
tableType:EXTERNAL_TABLE)
```