1.建表语句
先看一下官方给的完整的见表语句：
CREATE [TEMPORARY] [EXTERNAL] TABLE [IF NOT EXISTS] [db_name.]table_name -- (Note: TEMPORARY available in Hive 0.14.0 and later)
[(col_name data_type [column_constraint_specification] [COMMENT col_comment], ... [constraint_specification])]
[COMMENT table_comment]
[PARTITIONED BY (col_name data_type [COMMENT col_comment], ...)]
[CLUSTERED BY (col_name, col_name, ...) [SORTED BY (col_name [ASC|DESC], ...)] INTO num_buckets BUCKETS]
[SKEWED BY (col_name, col_name, ...) -- (Note: Available in Hive 0.10.0 and later)]
ON ((col_value, col_value, ...), (col_value, col_value, ...), ...)
[STORED AS DIRECTORIES]
[
[ROW FORMAT row_format]
[STORED AS file_format]
| STORED BY 'storage.handler.class.name' [WITH SERDEPROPERTIES (...)] -- (Note: Available in Hive 0.6.0 and later)
]
[LOCATION hdfs_path]
[TBLPROPERTIES (property_name=property_value, ...)] -- (Note: Available in Hive 0.6.0 and later)
[AS select_statement]; -- (Note: Available in Hive 0.5.0 and later; not supported for external tables)
CREATE [TEMPORARY] [EXTERNAL] TABLE [IF NOT EXISTS] [db_name.]table_name
LIKE existing_table_or_view_name
[LOCATION hdfs_path];
data_type
: primitive_type
| array_type
| map_type
| struct_type
| union_type -- (Note: Available in Hive 0.7.0 and later)
primitive_type
: TINYINT
| SMALLINT
| INT
| BIGINT
| BOOLEAN
| FLOAT
| DOUBLE
| DOUBLE PRECISION -- (Note: Available in Hive 2.2.0 and later)
| STRING
| BINARY -- (Note: Available in Hive 0.8.0 and later)
| TIMESTAMP -- (Note: Available in Hive 0.8.0 and later)
| DECIMAL -- (Note: Available in Hive 0.11.0 and later)
| DECIMAL(precision, scale) -- (Note: Available in Hive 0.13.0 and later)
| DATE -- (Note: Available in Hive 0.12.0 and later)
| VARCHAR -- (Note: Available in Hive 0.12.0 and later)
| CHAR -- (Note: Available in Hive 0.13.0 and later)
array_type
: ARRAY < data_type >
map_type
: MAP < primitive_type, data_type >
struct_type
: STRUCT < col_name : data_type [COMMENT col_comment], ...>
union_type
: UNIONTYPE < data_type, data_type, ... > -- (Note: Available in Hive 0.7.0 and later)
row_format
: DELIMITED [FIELDS TERMINATED BY char [ESCAPED BY char]] [COLLECTION ITEMS TERMINATED BY char]
[MAP KEYS TERMINATED BY char] [LINES TERMINATED BY char]
[NULL DEFINED AS char] -- (Note: Available in Hive 0.13 and later)
| SERDE serde_name [WITH SERDEPROPERTIES (property_name=property_value, property_name=property_value, ...)]
file_format:
: SEQUENCEFILE
| TEXTFILE -- (Default, depending on hive.default.fileformat configuration)
| RCFILE -- (Note: Available in Hive 0.6.0 and later)
| ORC -- (Note: Available in Hive 0.11.0 and later)
| PARQUET -- (Note: Available in Hive 0.13.0 and later)
| AVRO -- (Note: Available in Hive 0.14.0 and later)
| JSONFILE -- (Note: Available in Hive 4.0.0 and later)
| INPUTFORMAT input_format_classname OUTPUTFORMAT output_format_classname
column_constraint_specification:
: [ PRIMARY KEY|UNIQUE|NOT NULL|DEFAULT [default_value]|CHECK [check_expression] ENABLE|DISABLE NOVALIDATE RELY/NORELY ]
default_value:
: [ LITERAL|CURRENT_USER()|CURRENT_DATE()|CURRENT_TIMESTAMP()|NULL ]
constraint_specification:
: [, PRIMARY KEY (col_name, ...) DISABLE NOVALIDATE RELY/NORELY ]
[, PRIMARY KEY (col_name, ...) DISABLE NOVALIDATE RELY/NORELY ]
[, CONSTRAINT constraint_name FOREIGN KEY (col_name, ...) REFERENCES table_name(col_name, ...) DISABLE NOVALIDATE
[, CONSTRAINT constraint_name UNIQUE (col_name, ...) DISABLE NOVALIDATE RELY/NORELY ]
[, CONSTRAINT constraint_name CHECK [check_expression] ENABLE|DISABLE NOVALIDATE RELY/NORELY ]
2.主键约束
其中关于主键约束的定义语句如下：
PRIMARY KEY (col_name, ...) DISABLE NOVALIDATE RELY/NORELY
在 Hive 中，可以使用 PRIMARY KEY 子句来定义主键约束，以确保表中的每行数据都具有唯一标识。主键约束要求表中的每个行都必须具有唯一的主键值，这可以通过在表中定义一个或多个列来实现。
当使用 PRIMARY KEY 子句定义主键约束时，可以使用 DISABLE NOVALIDATE 子句来禁用主键约束的验证。
CREATE TABLE my_table (
id INT,
name STRING,
PRIMARY KEY (id) DISABLE NOVALIDATE
);
注意： 因为 Hive 目前对主键约束支持的不是很完善，现在暂不支持默认的 ENABLE 和 VALIDATE，所以定义主键约束时，「必须带上 DISABLE NOVALIDATE」，不然会报如下错误：
SemanticException [Error 10326]: Invalid Constraint syntax ENABLE/ENFORCED feature not supported yet. Please use DISABLE/NOT ENFORCED instead. (state=42000,code=1032
既然需要禁用主键约束，那么创建一个无法约束的主键意义何在呢？
3.主键约束的意义
对于唯一约束，目前 Hive 是不支持的，但是其他一些约束是支持的。
进入 HIVE-16575 的版本信息，会看到如下说明：
虽然 Hive 不支持主键与外键约束，但它们将被存储并可以使用 RELY 进行重写/优化。
定义主键约束时，RELY/NORELY 是可选的，缺省为 RELY。
如果一个约束指定 RELY，也就是希望 HIVE 基于代价的优化器 CBO（Cost-Based Optimizer）使用约束信息来获得更好的统计信息，得到更好的执行计划。
具体到主键约束，一般主键常用于多表关联查询，利用主键约束，可以删除不必要的连接（ JOIN ELIMINATION），得到更好的执行计划。
