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

Syntax

:::tip

The TABLE can either be a in your database (in which case you would pass the table’s name), or 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

ratings;

SELECT * FROM ratings;

Specific columns

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

Example:

SELECT movieId, rating FROM ratings;

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 movieId, (100 - rating)*2, rating > 3.5 good
FROM ratings;

The result of rating > 3.5 is a boolean. The column will be named good and take values true or false.

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 movieId alias1, rating alias2
FROM ratings

Aggregation

:::info

Supported aggregation functions are listed on the aggregation reference.

:::

Aggregation by group

QuestDB evaluates aggregation functions without need for traditional GROUP BY. Use 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.

SELECT movieId, avg(rating), count()
FROM ratings;

SELECT movieId, avg(rating), count()
FROM ratings
GROUP BY movieId;

Aggregation arithmetic

Aggregation functions can be used in arithmetic expressions. The following computes mid of rating values for every movie.

SELECT movieId, (min(rating) + max(rating))/2 mid, count() count
FROM ratings;

:::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(value/2) is going to execute considerably slower than min(value)/2 although both alternative will return the same result

:::

Supported clauses

QuestDB supports the following standard SQL clauses within SELECT statements.

CASE

Conditional results based on expressions.

Syntax

:::info

For more information, please refer to the CASE reference

:::

CAST

Convert values and expression between types.

Syntax

:::info

For more information, please refer to the CAST reference

:::

DISTINCT

Returns distinct values of the specified column(s).

Syntax

:::info

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

:::info

For more information, please refer to the FILL reference.

:::

JOIN

Join tables based on a key or timestamp.

Syntax

:::info

For more information, please refer to the JOIN reference

:::

LIMIT

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

Syntax

:::info

For more information, please refer to the LIMIT reference.

:::

ORDER BY

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

Syntax

:::info

For more information, please refer to the ORDER BY reference

:::

UNION

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

Syntax

:::info

For more information, please refer to the UNION reference

:::

WHERE

Filters query results

Syntax

:::info

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 BY

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

Syntax

:::info

For more information, please refer to the LATEST BY 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

:::info

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 BY 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

:::info

For more information, refer to the TIMESTAMP reference

:::