HCatalog - Alter Table

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This chapter explains how to alter the attributes of a table such as changing its table name, changing column names, adding columns, and deleting or replacing columns.

Alter Table Statement

You can use the ALTER TABLE statement to alter a table in Hive.

Syntax

The statement takes any of the following syntaxes based on what attributes we wish to modify in a table.

ALTER TABLE name RENAME TO new_name
ALTER TABLE name ADD COLUMNS (col_spec[, col_spec ...])
ALTER TABLE name DROP [COLUMN] column_name
ALTER TABLE name CHANGE column_name new_name new_type
ALTER TABLE name REPLACE COLUMNS (col_spec[, col_spec ...])

Some of the scenarios are explained below.

Rename To… Statement

The following query renames a table from employee to emp.

./hcat –e "ALTER TABLE employee RENAME TO emp;"

Change Statement

The following table contains the fields of employee table and it shows the fields to be changed (in bold).

Field Name Convert from Data Type Change Field Name Convert to Data Type
eid int eid int
name String ename String
salary Float salary Double
designation String designation String

The following queries rename the column name and column data type using the above data −

./hcat –e "ALTER TABLE employee CHANGE name ename String;"
./hcat –e "ALTER TABLE employee CHANGE salary salary Double;"

Add Columns Statement

The following query adds a column named dept to the employee table.

./hcat –e "ALTER TABLE employee ADD COLUMNS (dept STRING COMMENT 'Department name');"

Replace Statement

The following query deletes all the columns from the employee table and replaces it with emp and name columns −

./hcat – e "ALTER TABLE employee REPLACE COLUMNS ( eid INT empid Int, ename STRING name String);"

Drop Table Statement

This chapter describes how to drop a table in HCatalog. When you drop a table from the metastore, it removes the table/column data and their metadata. It can be a normal table (stored in metastore) or an external table (stored in local file system); HCatalog treats both in the same manner, irrespective of their types.

The syntax is as follows −

DROP TABLE [IF EXISTS] table_name;

The following query drops a table named employee −

./hcat –e "DROP TABLE IF EXISTS employee;"

On successful execution of the query, you get to see the following response −

OK
Time taken: 5.3 seconds
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