The simple SELECT clause is the main SQL syntax to read and process
existing data.
When used as a stand-alone statement, the simple SELECT clause is
also called "the SELECT statement". However, it is also a
selection clause that can be combined
with other constructs to form more complex selection queries.
Synopsis
The simple SELECT clause also has other applications not covered here, such as executing functions like SELECT current_timestamp();.
Required privileges
The user must have the SELECT privilege on the tables used as operands.
Parameters
| Parameter | Description |
|---|---|
DISTINCT or ALL |
See Eliminate Duplicate Rows. |
DISTINCT ON ( a_expr [, ...] ) |
DISTINCT ON followed by a list of scalar expressions within parentheses. See Eliminate Duplicate Rows. |
target_elem |
A scalar expression to compute a column in each result row, or * to automatically retrieve all columns from the FROM clause.If target_elem contains an aggregate function, a GROUP BY clause can be used to further control the aggregation. |
table_ref |
The table expression you want to retrieve data from. Using two or more table expressions in the FROM sub-clause, separated with a comma, is equivalent to a CROSS JOIN expression. |
AS OF SYSTEM TIME timestamp |
Retrieve data as it existed as of timestamp. Note: Because AS OF SYSTEM TIME returns historical data, your reads might be stale. |
WHERE a_expr |
Only retrieve rows that return TRUE for a_expr, which must be a scalar expression that returns Boolean values using columns (e.g., <column> = <value>). |
GROUP BY a_expr |
When using aggregate functions in target_elem or HAVING, list the column groupings after GROUP BY. |
HAVING a_expr |
Only retrieve aggregate function groups that return TRUE for a_expr, which must be a scalar expression that returns Boolean values using an aggregate function (e.g., <aggregate function> = <value>). HAVING works like the WHERE clause, but for aggregate functions. |
WINDOW window_definition_list |
A list of window functions definitions. |
Eliminate duplicate rows
The DISTINCT subclause specifies to remove duplicate rows.
By default, or when ALL is specified, SELECT returns all the rows
selected, without removing duplicates. When DISTINCT is specified,
duplicate rows are eliminated.
Without ON, two rows are considered duplicates if they are equal on
all the results computed by SELECT.
With ON, two rows are considered duplicates if they are equal only
using the scalar expressions listed with ON. When two rows are considered duplicates according to DISTINCT ON, the values from the first FROM row in the order specified by ORDER BY are used to compute the remaining target expressions. If ORDER BY is not specified, CockroachDB will pick any one of the duplicate rows as first row, non-deterministically.
Examples
Choose columns
Retrieve specific columns
Retrieve specific columns by naming them in a comma-separated list:
> SELECT id, name, balance
FROM accounts;
+----+-----------------------+---------+
| id | name | balance |
+----+-----------------------+---------+
| 1 | Bjorn Fairclough | 1200 |
| 2 | Bjorn Fairclough | 2500 |
| 3 | Arturo Nevin | 250 |
[ truncated ]
+----+-----------------------+---------+
Retrieve all columns
Retrieve all columns by using *:
> SELECT *
FROM accounts;
+----+-----------------------+---------+----------+--------------+
| id | name | balance | type | state_opened |
+----+-----------------------+---------+----------+--------------+
| 1 | Bjorn Fairclough | 1200 | checking | AL |
| 2 | Bjorn Fairclough | 2500 | savings | AL |
| 3 | Arturo Nevin | 250 | checking | AK |
[ truncated ]
+----+-----------------------+---------+----------+--------------+
Filter rows
Filter on a single condition
Filter rows with expressions that use columns and return Boolean values in the WHERE clause:
> SELECT name, balance
FROM accounts
WHERE balance < 300;
+------------------+---------+
| name | balance |
+------------------+---------+
| Arturo Nevin | 250 |
| Akbar Jinks | 250 |
| Andrea Maas | 250 |
+------------------+---------+
Filter on multiple conditions
To use multiple WHERE filters join them with AND or OR. You can also create negative filters with NOT:
> SELECT *
FROM accounts
WHERE balance > 2500 AND NOT type = 'checking';
+----+-------------------+---------+---------+--------------+
| id | name | balance | type | state_opened |
+----+-------------------+---------+---------+--------------+
| 4 | Tullia Romijnders | 3000 | savings | AK |
| 62 | Ruarc Mathews | 3000 | savings | OK |
+----+-------------------+---------+---------+--------------+
Select distinct rows
Columns without the Primary Key or Unique constraints can have multiple instances of the same value:
> SELECT name
FROM accounts
WHERE state_opened = 'VT';
+----------------+
| name |
+----------------+
| Sibylla Malone |
| Sibylla Malone |
+----------------+
Using DISTINCT, you can remove all but one instance of duplicate values from your retrieved data:
> SELECT DISTINCT name
FROM accounts
WHERE state_opened = 'VT';
+----------------+
| name |
+----------------+
| Sibylla Malone |
+----------------+
Filter values with a list
Using WHERE <column> IN (<comma separated list of values>) performs an OR search for listed values in the specified column:
> SELECT name, balance, state_opened
FROM accounts
WHERE state_opened IN ('AZ', 'NY', 'WA');
+-----------------+---------+--------------+
| name | balance | state_opened |
+-----------------+---------+--------------+
| Naseem Joossens | 300 | AZ |
| Aygün Sanna | 900 | NY |
| Carola Dahl | 800 | NY |
| Edna Barath | 750 | WA |
| Edna Barath | 2200 | WA |
+-----------------+---------+--------------+
Rename columns in output
Instead of outputting a column's name in the retrieved table, you can change its label using AS:
> SELECT name AS NY_accounts, balance
FROM accounts
WHERE state_opened = 'NY';
+-------------+---------+
| NY_accounts | balance |
+-------------+---------+
| Aygün Sanna | 900 |
| Carola Dahl | 800 |
+-------------+---------+
This does not change the name of the column in the table. To do that, use RENAME COLUMN.
Search for string values
Search for partial string matches in columns using LIKE, which supports the following wildcard operators:
%matches 0 or more characters._matches exactly 1 character.
For example:
> SELECT id, name, type
FROM accounts
WHERE name LIKE 'Anni%';
+----+----------------+----------+
| id | name | type |
+----+----------------+----------+
| 58 | Annibale Karga | checking |
| 59 | Annibale Karga | savings |
+----+----------------+----------+
Aggregate functions
Aggregate functions perform calculations on retrieved rows.
Perform aggregate function on entire column
By using an aggregate function as a target_elem, you can perform the calculation on the entire column.
> SELECT MIN(balance)
FROM accounts;
+--------------+
| MIN(balance) |
+--------------+
| 250 |
+--------------+
You can also use the retrieved value as part of an expression. For example, you can use the result in the WHERE clause to select additional rows that were not part of the aggregate function itself:
> SELECT id, name, balance
FROM accounts
WHERE balance = (
SELECT
MIN(balance)
FROM accounts
);
+----+------------------+---------+
| id | name | balance |
+----+------------------+---------+
| 3 | Arturo Nevin | 250 |
| 10 | Henrik Brankovic | 250 |
| 26 | Odalys Ziemniak | 250 |
| 35 | Vayu Soun | 250 |
+----+------------------+---------+
Perform aggregate function on retrieved rows
By filtering the statement, you can perform the calculation only on retrieved rows:
> SELECT SUM(balance)
FROM accounts
WHERE state_opened IN ('AZ', 'NY', 'WA');
+--------------+
| SUM(balance) |
+--------------+
| 4950 |
+--------------+
Filter columns fed into aggregate functions
You can use FILTER (WHERE <Boolean expression>) in the target_elem to filter which rows are processed by an aggregate function; those that return FALSE or NULL for the FILTER clause's Boolean expression are not fed into the aggregate function:
> SELECT count(*) AS unfiltered, count(*) FILTER (WHERE balance > 1500) AS filtered FROM accounts;
+------------+----------+
| unfiltered | filtered |
+------------+----------+
| 84 | 14 |
+------------+----------+
Create aggregate groups
Instead of performing aggregate functions on an the entire set of retrieved rows, you can split the rows into groups and then perform the aggregate function on each of them.
When creating aggregate groups, each column used as a target_elem must be included in GROUP BY.
For example:
> SELECT state_opened AS state, SUM(balance) AS state_balance
FROM accounts
WHERE state_opened IN ('AZ', 'NY', 'WA')
GROUP BY state_opened;
+-------+---------------+
| state | state_balance |
+-------+---------------+
| AZ | 300 |
| NY | 1700 |
| WA | 2950 |
+-------+---------------+
Filter aggregate groups
To filter aggregate groups, use HAVING, which is the equivalent of the WHERE clause for aggregate groups, which must evaluate to a Boolean value.
For example:
> SELECT state_opened, AVG(balance) as avg
FROM accounts
GROUP BY state_opened
HAVING AVG(balance) BETWEEN 1700 AND 50000;
+--------------+---------+
| state_opened | avg |
+--------------+---------+
| AR | 3700.00 |
| UT | 1750.00 |
| OH | 2500.00 |
| AL | 1850.00 |
+--------------+---------+
Use aggregate functions in having clause
Aggregate functions can also be used in the HAVING clause without needing to be included as a target_elem.
For example:
> SELECT name, state_opened
FROM accounts
WHERE state_opened in ('LA', 'MO')
GROUP BY name, state_opened
HAVING COUNT(name) > 1;
+----------------+--------------+
| name | state_opened |
+----------------+--------------+
| Yehoshua Kleid | MO |
+----------------+--------------+
Select historical data (time-travel)
CockroachDB lets you find data as it was stored at a given point in
time using AS OF SYSTEM TIME with various supported
formats. This can be also advantageous for
performance. For more details, see AS OF SYSTEM
TIME.
Advanced uses of SELECT clauses
CockroachDB supports numerous ways to combine results from SELECT
clauses together.
See Selection Queries for details. A few examples follow.
Sorting and limiting query results
To order the results of a SELECT clause or limit the number of rows
in the result, you can combine it with ORDER BY or LIMIT /
OFFSET to form a selection query or
subquery.
See Ordering Query Results and Limiting Query Results for more details.
ORDER BY is not included in a query, rows are not sorted by any consistent criteria. Instead, CockroachDB returns them as the coordinating node receives them.Also, CockroachDB sorts
NULL values first with ASC and last with DESC. This differs from PostgreSQL, which sorts NULL values last with ASC and first with DESC.Combining results from multiple queries
Results from two or more queries can be combined together as follows:
- Using join expressions to combine rows according to conditions on specific columns.
- Using set operations to combine rows using inclusion/exclusion rules.
See also
- Scalar Expressions
- Selection Clauses
- Set Operations
- Table Expressions
- Ordering Query Results
- Limiting Query Results
- SQL Performance Best Practices
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