Synopsis

Use the SELECT statement to retrieve (part of) rows of specified columns that meet a given condition from a table. It specifies the columns to be retrieved, the name of the table, and the condition each selected row must satisfy.

Syntax

Diagram

select

SELECTDISTINCT*,column_nameFROMtable_nameWHEREwhere_expressionALLOW FILTERINGIFif_expressionORDER BYorder_expressionLIMITlimit_expressionOFFSEToffset_expression

order_expression

(,column_nameASCDESC)

Grammar

select ::= SELECT [ DISTINCT ] { * | column_name [ , column_name ... ] }
               FROM table_name
               [ WHERE where_expression ]
               [ IF where_expression ]
               [ ORDER BY order_expression ]
               [ LIMIT limit_expression ] [ OFFSET offset_expression ]

order_expression ::= ( { column_name [ ASC | DESC ] } [ , ... ] )

Where

  • table_name and column_name are identifiers (table_name may be qualified with a keyspace name).
  • limit_expression is an integer literal (or a bind variable marker for prepared statements).
  • Restrictions for where_expression are discussed in the Semantics section below.
  • See Expressions for more information on syntax rules.

Semantics

  • An error is raised if the specified table_name does not exist.
  • SELECT DISTINCT can only be used for partition columns or static columns.
  • * means all columns of the table will be retrieved.
  • LIMIT clause sets the maximum number of results (rows) to be returned.
  • OFFSET clause sets the number of rows to be skipped before returning results.

ORDER BY clause

  • The ORDER BY clause sets the order for the returned results.
  • Only clustering columns are allowed in the order_expression.
  • For a given column, DESC means descending order and ASC or omitted means ascending order.
  • Currently, only two overall orderings are allowed, the clustering order from the CREATE TABLE statement (forward scan) or its opposite (reverse scan).

WHERE clause

  • The where_expression must evaluate to boolean values.

  • The where_expression can specify conditions on any columns including partition, clustering, and regular columns.

  • The where_expression has a restricted list of operators.

    • Only =, !=, IN and NOT IN operators can be used for conditions on partition columns.
    • Only operators =, !=, <, <=, >, >=, IN and NOT IN can be used for conditions on clustering and regular columns.
    • Only IN operator can be used for conditions on tuples of clustering columns.

IF clause

  • The if_expression must evaluate to boolean values.
  • The if_expression supports any combinations of all available boolean and logical operators.
  • The if_expression can only specify conditions for non-primary-key columns although it can used on a key column of a secondary index.
  • While WHERE condition is used to generate efficient query plan, the IF condition is not. ALL rows that satisfy WHERE condition will be read from the database before the IF condition is used to filter unwanted data. In the following example, although the two queries yield the same result set, SELECT with WHERE clause will use INDEX-SCAN while SELECT with IF clause will use FULL-SCAN.
SELECT * FROM a_table WHERE key = 'my_key';
SELECT * FROM a_table IF key = 'my_key';

Note

While the where clause allows a wide range of operators, the exact conditions used in the where clause have significant performance considerations (especially for large datasets). Some best practices are:

  • Use equality conditions on all partition columns (to fix the value of the partition key).
  • Use comparison operators on the clustering columns (tighter restrictions are more valuable for left-most clustering columns).
  • Generally, the closer a column is to the beginning of the primary key, the higher the performance gain for setting tighter restrictions on it.

Ideally, these performance considerations should be taken into account when creating the table schema.

Examples

Select all rows from a table

ycqlsh:example> CREATE TABLE employees(department_id INT,
                                      employee_id INT,
                                      dept_name TEXT STATIC,
                                      employee_name TEXT,
                                      PRIMARY KEY(department_id, employee_id));
ycqlsh:example> INSERT INTO employees(department_id, employee_id, dept_name, employee_name)
                   VALUES (1, 1, 'Accounting', 'John');
ycqlsh:example> INSERT INTO employees(department_id, employee_id, dept_name, employee_name)
                   VALUES (1, 2, 'Accounting', 'Jane');
ycqlsh:example> INSERT INTO employees(department_id, employee_id, dept_name, employee_name)
                   VALUES (1, 3, 'Accounting', 'John');
ycqlsh:example> INSERT INTO employees(department_id, employee_id, dept_name, employee_name)
                   VALUES (2, 1, 'Marketing', 'Joe');
ycqlsh:example> SELECT * FROM employees;
 department_id | employee_id | dept_name  | employee_name
---------------+-------------+------------+---------------
             1 |           1 | Accounting |          John
             1 |           2 | Accounting |          Jane
             1 |           3 | Accounting |          John
             2 |           1 |  Marketing |           Joe

Select with limit

ycqlsh:example> SELECT * FROM employees LIMIT 2;
 department_id | employee_id | dept_name  | employee_name
---------------+-------------+------------+---------------
             1 |           1 | Accounting |          John
             1 |           2 | Accounting |          Jane

Select with offset

ycqlsh:example> SELECT * FROM employees LIMIT 2 OFFSET 1;
 department_id | employee_id | dept_name  | employee_name
---------------+-------------+------------+---------------
             1 |           2 | Accounting |          Jane
             1 |           3 | Accounting |          John

Select distinct values

ycqlsh:example> SELECT DISTINCT dept_name FROM employees;
 dept_name
------------
 Accounting
  Marketing

Select with a condition on the partitioning column

ycqlsh:example> SELECT * FROM employees WHERE department_id = 2;
 department_id | employee_id | dept_name | employee_name
---------------+-------------+-----------+---------------
             2 |           1 | Marketing |           Joe

Select with condition on the clustering column

ycqlsh:example> SELECT * FROM employees WHERE department_id = 1 AND employee_id <= 2;
 department_id | employee_id | dept_name  | employee_name
---------------+-------------+------------+---------------
             1 |           1 | Accounting |          John
             1 |           2 | Accounting |          Jane

Select with condition on a regular column, using WHERE clause

ycqlsh:example> SELECT * FROM employees WHERE department_id = 1 AND employee_name = 'John' ALLOW FILTERING;
 department_id | employee_id | dept_name  | employee_name
---------------+-------------+------------+---------------
             1 |           1 | Accounting |          John
             1 |           3 | Accounting |          John

Select with condition on a regular column, using IF clause

ycqlsh:example> SELECT * FROM employees WHERE department_id = 1 IF employee_name != 'John';
 department_id | employee_id | dept_name  | employee_name
---------------+-------------+------------+---------------
             1 |           2 | Accounting |          Jane

Select with ORDER BY clause

ycqlsh:example> CREATE TABLE sensor_data(device_id INT,
                                        sensor_id INT,
                                        ts TIMESTAMP,
                                        value TEXT,
                                        PRIMARY KEY((device_id), sensor_id, ts)) WITH CLUSTERING ORDER BY (sensor_id ASC, ts DESC);
ycqlsh:example> INSERT INTO sensor_data(device_id, sensor_id, ts, value)
                   VALUES (1, 1, '2018-1-1 12:30:30 UTC', 'a');
ycqlsh:example> INSERT INTO sensor_data(device_id, sensor_id, ts, value)
                   VALUES (1, 1, '2018-1-1 12:30:31 UTC', 'b');
ycqlsh:example> INSERT INTO sensor_data(device_id, sensor_id, ts, value)
                   VALUES (1, 2, '2018-1-1 12:30:30 UTC', 'x');
ycqlsh:example> INSERT INTO sensor_data(device_id, sensor_id, ts, value)
                   VALUES (1, 2, '2018-1-1 12:30:31 UTC', 'y');

Reverse scan, opposite of the table's clustering order.

ycqlsh:example> SELECT * FROM sensor_data WHERE device_id = 1 ORDER BY sensor_id DESC, ts ASC;
 device_id | sensor_id | ts                              | value
-----------+-----------+---------------------------------+-------
         1 |         2 | 2018-01-01 12:30:30.000000+0000 |     x
         1 |         2 | 2018-01-01 12:30:31.000000+0000 |     y
         1 |         1 | 2018-01-01 12:30:30.000000+0000 |     a
         1 |         1 | 2018-01-01 12:30:31.000000+0000 |     b

Forward scan, same as a SELECT without an ORDER BY clause.

ycqlsh:example> SELECT * FROM sensor_data WHERE device_id = 1 ORDER BY sensor_id ASC, ts DESC;
 device_id | sensor_id | ts                              | value
-----------+-----------+---------------------------------+-------
         1 |         1 | 2018-01-01 12:30:31.000000+0000 |     b
         1 |         1 | 2018-01-01 12:30:30.000000+0000 |     a
         1 |         2 | 2018-01-01 12:30:31.000000+0000 |     y
         1 |         2 | 2018-01-01 12:30:30.000000+0000 |     x

Other orderings are not allowed.

ycqlsh:example> SELECT * FROM sensor_data WHERE device_id = 1 ORDER BY sensor_id ASC, ts ASC;
InvalidRequest: Unsupported order by relation
SELECT * FROM sensor_data WHERE device_id = 1 ORDER BY sensor_id ASC, ts ASC;
                                                        ^^^^^^^^^^^^^^^^^^^^^

See also