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Select Syntax

SELECT [ALL | DISTINCT] select_expr, select_expr, ...
FROM table_reference
[WHERE where_condition] 
[GROUP BY col_list]
[CLUSTER BY col_list
  | [DISTRIBUTE BY col_list] [SORT BY col_list]
]
[LIMIT number]
  • A SELECT statement can be part of a union query or a subquery of another query.
  • table_reference indicates 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. 
    SELECT * FROM t1

    WHERE Clause

    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 does not support IN, EXISTS or subqueries in the WHERE clause. 
    SELECT * FROM sales WHERE amount > 10 AND region = "US"

    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. 
    hive> SELECT col1, col2 FROM t1
    1 3
    1 3
    1 4
    2 5
hive> SELECT DISTINCT col1, col2 FROM t1
    1 3
    1 4
    2 5
hive> SELECT DISTINCT col1 FROM t1
    1
    2

    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. 
        SELECT page_views.* 
    FROM page_views 
    WHERE page_views.date >= '2008-03-01' AND page_views.date <= '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:

    SELECT page_views.* 
    FROM page_views JOIN dim_users 
      ON (page_views.user_id = dim_users.id AND page_views.date >= '2008-03-01' AND page_views.date <= '2008-03-31')
  • See also Sort By / Cluster By / Distribute By / Order By

    HAVING Clause

    Hive currently does not support HAVING clause. A similar effect can be achieved by using a subquery. For example, 
    SELECT col1 FROM t1 GROUP BY col1 HAVING SUM(col2) > 10
    can also be expressed as 
    SELECT col1 FROM (SELECT col1, SUM(col2) AS col2sum FROM t1 GROUP BY col1) t2 WHERE t2.col2sum > 10

    LIMIT Clause

    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. 
    SELECT * FROM t1 LIMIT 5
  • Top k queries. The following query returns the top 5 sales records wrt amount. 
    SET mapred.reduce.tasks = 1
SELECT * FROM sales SORT BY amount DESC LIMIT 5

    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. 
    SELECT {{(ds|hr)?+.+}} FROM sales
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