CREATE TABLE creates a table with the given name. An error is thrown if a table or view with the same name already exists. You can use IF NOT EXISTS to skip the error.
The EXTERNAL keyword lets you create a table and provide a LOCATION so that Hive does not use a default location for this table. This comes in handy if you already have data generated. When dropping an EXTERNAL table, data in the table is NOT deleted from the file system.
You can create tables with custom SerDe or using native SerDe. A native SerDe is used if ROW FORMAT is not specified or ROW FORMAT DELIMITED is specified. You can use the DELIMITED clause to read delimited files. Use the SERDE clause to create a table with custom SerDe. Refer to SerDe section of the User Guide for more information on SerDe.
You must specify a list of a columns for tables that use a native SerDe. Refer to the Types part of the User Guide for the allowable column types. A list of columns for tables that use a custom SerDe may be specified but Hive will query the SerDe to determine the actual list of columns for this table.
Use STORED AS TEXTFILE if the data needs to be stored as plain text files. Use STORED AS SEQUENCEFILE if the data needs to be compressed. Please read more about Hive/CompressedStorage if you are planning to keep data compressed in your Hive tables. Use INPUTFORMAT and OUTPUTFORMAT to specify the name of a corresponding InputFormat and OutputFormat class as a string literal, e.g. 'org.apache.hadoop.hive.contrib.fileformat.base64.Base64TextInputFormat'.
Partitioned tables can be created using the PARTITIONED BY clause. A table can have one or more partition columns and a separate data directory is created for each distinct value combination in the partition columns. Further, tables or partitions can be bucketed using CLUSTERED BY columns, and data can be sorted within that bucket via SORT BY columns. This can improve performance on certain kinds of queries.
Table names and column names are case insensitive but SerDe and property names are case sensitive. Table and column comments are string literals (single-quoted). The TBLPROPERTIES clause allows you to tag the table definition with your own metadata key/value pairs.
A create table example:
The statement above creates the page_view table with viewTime, userid, page_url, referrer_url, and ip columns (including comments). The table is also partitioned and data is stored in sequence files. The data format in the files is assumed to be field-delimited by ctrl-A and row-delimited by newline.
Tables can also be created and populated by the results of a query in one create-table-as-select (CTAS) statement. The table created by CTAS is atomic, meaning that the table is not seen by other users until all the query results are populated. So other users will either see the table with the complete results of the query or will not see the table at all.
There are two parts in CTAS, the SELECT part can be any SELECT statement supported by HiveQL. The CREATE part of the CTAS takes the resulting schema from the SELECT part and creates the target table with other table properties such as the SerDe and storage format. The only restrictions in CTAS is that the target table cannot be a partitioned table (nor can it be an external table).
This example CTAS statement creates the target table new_key_value_store with the schema (new_key DOUBLE, key_value_pair STRING) derived from the results of the SELECT statement. If the SELECT statement does not specify column aliases, the column names will be automatically assigned to _col0, _col1, and _col2 etc. In addition, the new target table is created using a specific SerDe and a storage format independent of the source tables in the SELECT statement.
Being able to select data from one table to another is one of the most powerful features of Hive. Hive handles the conversion of the data from the source format to the destination format as the query is being executed!
In the example above, the page_view table is bucketed (clustered by) userid and within each bucket the data is sorted in increasing order of viewTime. Such an organization allows the user to do efficient sampling on the clustered column - in this case userid. The sorting property allows internal operators to take advantage of the better-known data structure while evaluating queries, also increasing efficiency. MAP KEYS and COLLECTION ITEMS keywords can be used if any of the columns are lists or maps.
The CLUSTERED BY and SORTED BY creation commands do not affect how data is inserted into a table -- only how it is read. This means that users must be careful to insert data correctly by specifying the number of reducers to be equal to the number of buckets, and using CLUSTER BY and SORT BY commands in their query. See Working with Bucketed tables to see how these are used.
Unless a table is specified as EXTERNAL it will be stored inside a folder specified by the configuration property hive.metastore.warehouse.dir. EXTERNAL tables points to any hdfs location for its storage. You still have to make sure that the data is format is specified to match the data.
The statement above creates a new empty_key_value_store table whose definition exactly matches the existing key_value_store in all particulars other than table name. The new table contains no rows.
Drop it like it is hot