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SELECT

  • Artikkeli

Applies to: check marked yes Databricks SQL check marked yes Databricks Runtime

Composes a result set from one or more table references. The SELECT clause can be part of a query which also includes common table expressions (CTE), set operations, and various other clauses.

Syntax

SELECT [ hints ] [ ALL | DISTINCT ] { named_expression | star_clause } [, ...]
  FROM table_reference [, ...]
  [ LATERAL VIEW clause ]
  [ WHERE clause ]
  [ GROUP BY clause ]
  [ HAVING clause]
  [ QUALIFY clause ]

named_expression
   expression [ column_alias ]

star_clause
   [ { table_name | view_name } . ] * [ except_clause ]

except_clause
   EXCEPT ( { column_name | field_name } [, ...] )

Parameters

  • hints

    Hints help the Azure Databricks optimizer make better planning decisions. Azure Databricks supports hints that influence selection of join strategies and repartitioning of the data.

  • ALL

    Select all matching rows from the table references. Enabled by default.

  • DISTINCT

    Select all matching rows from the table references after removing duplicates in results.

  • named_expression

    An expression with an optional assigned name.

    • expression

      A combination of one or more values, operators, and SQL functions that evaluates to a value.

    • column_alias

      An optional column identifier naming the expression result. If no column_alias is provided Databricks SQL derives one.

  • star_clause

    A shorthand to name all the referenceable columns in the FROM clause or a specific table reference’s columns or fields in the FROM clause.

  • table_reference

    A source of input for the SELECT. This input reference can be turned into a streaming reference by using the STREAM keyword prior to the reference.

  • LATERAL VIEW

    Used in conjunction with generator functions such as EXPLODE, which generates a virtual table containing one or more rows. LATERAL VIEW applies the rows to each original output row.

    In Databricks SQL, and starting with Databricks Runtime 12.2 this clause is deprecated. You should invoke a table valued generator function as a table_reference.

  • WHERE

    Filters the result of the FROM clause based on the supplied predicates.

  • GROUP BY

    The expressions that are used to group the rows. This is used in conjunction with aggregate functions (MIN, MAX, COUNT, SUM, AVG) to group rows based on the grouping expressions and aggregate values in each group. When a FILTER clause is attached to an aggregate function, only the matching rows are passed to that function.

  • HAVING

    The predicates by which the rows produced by GROUP BY are filtered. The HAVING clause is used to filter rows after the grouping is performed. If you specify HAVING without GROUP BY, it indicates a GROUP BY without grouping expressions (global aggregate).

  • QUALIFY

    The predicates that are used to filter the results of window functions. To use QUALIFY, at least one window function is required to be present in the SELECT list or the QUALIFY clause.

Select on Delta table

In addition to the standard SELECT options, Delta tables support the time travel options described in this section. For details, see Work with Delta Lake table history.

AS OF syntax

table_identifier TIMESTAMP AS OF timestamp_expression

table_identifier VERSION AS OF version
  • timestamp_expression can be any one of:
    • '2018-10-18T22:15:12.013Z', that is, a string that can be cast to a timestamp
    • cast('2018-10-18 13:36:32 CEST' as timestamp)
    • '2018-10-18', that is, a date string
    • current_timestamp() - interval 12 hours
    • date_sub(current_date(), 1)
    • Any other expression that is or can be cast to a timestamp
  • version is a long value that can be obtained from the output of DESCRIBE HISTORY table_spec.

Neither timestamp_expression nor version can be subqueries.

Example

> SELECT * FROM events TIMESTAMP AS OF '2018-10-18T22:15:12.013Z'

> SELECT * FROM events VERSION AS OF 123

@ syntax

Use the @ syntax to specify the timestamp or version. The timestamp must be in yyyyMMddHHmmssSSS format. You can specify a version after @ by prepending a v to the version. For example, to query version 123 for the table events, specify events@v123.

Example

> SELECT * FROM events@20190101000000000

> SELECT * FROM events@v123

Examples

-- select all referencable columns from all tables
> SELECT * FROM VALUES(1, 2) AS t1(c1, c2), VALUES(3, 4) AS t2(c3, c4);
  1   2   3   4

-- select all referencable columns from one table
> SELECT t2.* FROM VALUES(1, 2) AS t1(c1, c2), VALUES(3, 4) AS t2(c3, c4);
  3   4

-- select all referencable columns from all tables except t2.c4
> SELECT * EXCEPT(c4) FROM VALUES(1, 2) AS t1(c1, c2), VALUES(3, 4) AS t2(c3, c4);
  1   2   3

-- select all referencable columns from a table, except a nested field.
> SELECT * EXCEPT(c2.b) FROM VALUES(1, named_struct('a', 2, 'b', 3)) AS t(c1, c2);
  1  { "a" : 2 }

-- Removing all fields results in an empty struct
> SELECT * EXCEPT(c2.b, c2.a) FROM VALUES(1, named_struct('a', 2, 'b', 3)) AS t(c1, c2);
  1  { }

-- Overlapping names result in an error
> SELECT * EXCEPT(c2, c2.a) FROM VALUES(1, named_struct('a', 2, 'b', 3)) AS t(c1, c2);
  Error: EXCEPT_OVERLAPPING_COLUMNS