SELECT keyword

SELECT allows you to specify a list of columns and expressions to be selected and evaluated from a table.

tip

Looking for SELECT best practices? Checkout our Maximize your SQL efficiency: SELECT best practices blog.

Syntax

Note: table can either a specified table in your database or passed forward as the result of a sub-query.

Simple select

All columns

QuestDB supports SELECT * FROM tablename. When selecting all, you can also omit most of the statement and pass the table name.

The two examples below are equivalent

QuestDB dialect

trades;

Traditional SQL equivalent

SELECT * FROM trades;

Specific columns

To select specific columns, replace * by the names of the columns you are interested in.

Example:

SELECT timestamp, symbol, side FROM trades;

Aliases

Using aliases allow you to give expressions or column names of your choice. You can assign an alias to a column or an expression by writing the alias name you want after that expression.

note

Alias names and column names must be unique.

SELECT timestamp, symbol,
    price AS rate,
    amount quantity
FROM trades;

Notice how you can use or omit the AS keyword.

Arithmetic expressions

SELECT is capable of evaluating multiple expressions and functions. You can mix comma separated lists of expressions with the column names you are selecting.

SELECT timestamp, symbol,
    price * 0.25 AS price25pct,
    amount > 10 AS over10
FROM trades

The result of amount > 10 is a boolean. The column will be named "over10" and take values true or false.

Boolean expressions

Supports AND/OR, NOT & XOR.

AND and OR

AND returns true if both operands are true, and false otherwise.

OR returns true if at least one of the operands is true.

SELECT
    (true AND false) AS this_will_return_false,
    (true OR false) AS this_will_return_true;

NOT

NOT inverts the truth value of the operand.

SELECT
    NOT (true AND false) AS this_will_return_true;

XOR

^ is the bitwise XOR operator. It applies only to the Long data type. Depending on what you need, you might prefer to cast the input and output to boolean values.

SELECT
    (1 ^ 1) AS will_return_0,
    (1 ^ 20) AS will_return_21,
    (true::int ^ false::long)::boolean AS will_return_true,
    (true::int ^ true::long)::boolean AS will_return_false;

Aggregation

Supported aggregation functions are listed on the aggregation reference.

Aggregation by group

QuestDB evaluates aggregation functions without need for traditional GROUP BY whenever there is a mix of column names and aggregation functions in a SELECT clause. You can have any number of discrete value columns and any number of aggregation functions. The three statements below are equivalent.

QuestDB dialect

SELECT symbol, avg(price), count()
FROM trades;

Traditional SQL equivalent

SELECT symbol, avg(price), count()
FROM trades
GROUP BY Symbol;

Traditional SQL equivalent with positional argument

SELECT symbol, avg(price), count()
FROM trades
GROUP BY 1;

Aggregation arithmetic

Aggregation functions can be used in arithmetic expressions. The following computes mid of prices for every symbol.

SELECT symbol, (min(price) + max(price))/2 mid, count() count
FROM trades;

tip

Whenever possible, it is recommended to perform arithmetic outside of aggregation functions as this can have a dramatic impact on performance. For example, min(price/2) is going to execute considerably more slowly than min(price)/2, although both return the same result.

Supported clauses

QuestDB supports the following standard SQL clauses within SELECT statements.

CASE

Conditional results based on expressions.

Syntax

For more information, please refer to the CASE reference

CAST

Convert values and expression between types.

Syntax

For more information, please refer to the CAST reference

DISTINCT

Returns distinct values of the specified column(s).

Syntax

For more information, please refer to the DISTINCT reference.

FILL

Defines filling strategy for missing data in aggregation queries. This function complements SAMPLE BY queries.

Syntax

For more information, please refer to the FILL reference.

JOIN

Join tables based on a key or timestamp.

Syntax

For more information, please refer to the JOIN reference

LIMIT

Specify the number and position of records returned by a query.

Syntax

For more information, please refer to the LIMIT reference.

ORDER BY

Orders the results of a query by one or several columns.

Syntax

For more information, please refer to the ORDER BY reference

UNION, EXCEPT & INTERSECT

Combine the results of two or more select statements. Can include or ignore duplicates.

Syntax

For more information, please refer to the UNION, EXCEPT & INTERSECT reference

WHERE

Filters query results

Syntax

QuestDB supports complex WHERE clauses along with type-specific searches. For more information, please refer to the WHERE reference. There are different syntaxes for text, numeric, or timestamp filters.

Additional time-series clauses

QuestDB augments SQL with the following clauses.

LATEST ON

Retrieves the latest entry by timestamp for a given key or combination of keys This function requires a designated timestamp.

Syntax

For more information, please refer to the LATEST ON reference.

SAMPLE BY

Aggregates time-series data into homogeneous time chunks. For example daily average, monthly maximum etc. This function requires a designated timestamp.

Syntax

For more information, please refer to the SAMPLE BY reference.

TIMESTAMP

Dynamically creates a designated timestamp on the output of a query. This allows to perform timestamp operations like SAMPLE BY or LATEST ON on tables which originally do not have a designated timestamp.

caution

The output query must be ordered by time. TIMESTAMP() does not check for order and using timestamp functions on unordered data may produce unexpected results.

Syntax

For more information, refer to the TIMESTAMP reference