- Select Syntax
- WHERE Clause
- ALL and DISTINCT Clauses
- Partition Based Queries
- HAVING Clause
- LIMIT Clause
- REGEX Column Specification
- More Select Syntax
- A SELECT statement can be part of a union query or a subquery of another query.
table_referenceindicates the input to the query. It can be a regular table, a view, a join construct or a subquery.
- Simple query. For example, the following query retrieves all columns and all rows from table t1.
The WHERE condition is a boolean expression. For example, the following query returns only those sales records which have an amount greater than 10 from the US region. Hive supports a number of operators and UDFs in the WHERE clause:
As of Hive 0.13 some types of subqueries are supported in the WHERE clause.
ALL and DISTINCT Clauses
The ALL and DISTINCT options specify whether duplicate rows should be returned. If none of these options are given, the default is ALL (all matching rows are returned). DISTINCT specifies removal of duplicate rows from the result set.
Partition Based Queries
In general, a SELECT query scans the entire table (other than for sampling). If a table created using the PARTITIONED BY clause, a query can do partition pruning and scan only a fraction of the table relevant to the partitions specified by the query. Hive currently does partition pruning if the partition predicates are specified in the WHERE clause or the ON clause in a JOIN. For example, if table page_views is partitioned on column date, the following query retrieves rows for just days between 2008-03-01 and 2008-03-31.
If a table page_views is joined with another table dim_users, you can specify a range of partitions in the ON clause as follows:
Hive added support for the HAVING clause in version 0.7.0. In older versions of Hive it is possible to achieve the same effect by using a subquery, e.g:
can also be expressed as
Limit indicates the number of rows to be returned. The rows returned are chosen at random. The following query returns 5 rows from t1 at random.
Top k queries. The following query returns the top 5 sales records wrt amount.
REGEX Column Specification
A SELECT statement can take regex-based column specification.
- We use java regex syntax. Try http://www.fileformat.info/tool/regex.htm for testing purposes.
- The following query select all columns except ds and hr.
More Select Syntax
See the following documents for additional syntax and features of SELECT statements: