Getting started with Looker

Best Practices

Documentation

Community

Training

Certification

Articles in this section

See more

dimension_group

Avatar
Looker Support
  • Updated
Follow

Usage

view: view_name {
  dimension_group: field_name { … }
}

Hierarchy

view
dimension_group

Accepts

A Looker identifier (to serve as the first part of the name for each dimension created by the dimension group)

Special Rules

Definition

The dimension_group parameter is used to create a set of time-based or duration-based dimensions all at once. You define the dimension group, and the dimension group will create a set of individual dimensions for different intervals or timeframes. For example, you can specify a dimension group of type: time based on a timestamp column, and the dimension group will create corresponding dimensions to express the data in time, date, week, hour, quarter, and year.

The form and function of the dimension group are different depending the type value of the dimension group:

Duration type

type: duration is used in conjunction with a dimension_group to calculate a set of interval-based duration dimensions.

The form of a dimension group of type: duration is:

dimension_group: dimension_group_name { type: duration sql_start: SQL expression ;; # often this is a single database column sql_end: SQL expression ;; # often this is a single database column intervals: [interval, interval, …] # valid intervals described below }

For dimension groups of type: duration:

  • The sql_start and sql_end parameters provide SQL expressions defining the start time and end time for the duration. See the Defining the start and end of a duration section on this page for details.

  • The intervals parameter specifies one or more interval units that should be used to measure the time difference. The possible choices are listed in the Interval options section on this page.

  • The duration values are floored to the nearest integer.

  • The datatype parameter is optional. If your dimension group is not based on a datetime you may specify an epoch, timestamp, date, or yyyymmdd format instead. For dimension groups of type: duration, the datatype parameter applies to both the sql_start and sql_end parameters, so be sure the sql_start and sql_end are both of the specified data type. The datatype parameter is described in greater detail in the Specifying the database datatype section on this page.

Although they are not listed here, many of the field-level parameters can be used with dimension groups as well.

As an example, if you have columns for enrollment_date and graduation_date, you can create a duration dimension group to see how much time students spent in school, calculated in week and year intervals:

dimension_group: enrolled { type: duration intervals: [week, year] sql_start: ${TABLE}.enrollment_date ;; sql_end: ${TABLE}.graduation_date ;; }

In the Explore UI, this would generate a dimension group called Duration Enrolled, with individual dimensions called Weeks Enrolled and Years Enrolled.

Interval options

The intervals parameter tells the dimension group which interval units it should use to measure the time difference between the sql_start time and the sql_end time. The intervals parameter is supported only for dimension groups of type: duration.

If intervals is not included, the dimension group will include all possible intervals.

The options for the intervals parameter are:

Interval Description Example Output
day Calculates a time difference in days. 9 days
hour Calculates a time difference in hours. 171 hours
minute Calculates a time difference in minutes. 10305 minutes
month Calculates a time difference in months. 3 months
quarter Calculates a time difference in quarters of the year. 2 quarters
second Calculates a time difference in seconds. 606770 seconds
week Calculates a time difference in weeks. 6 weeks
year Calculates a time difference in years. 2 years

Defining the start and end of a duration

For dimension groups of type: duration, the sql_start and sql_end parameters provide the start and end information used to calculate a time difference. These fields can take any valid SQL expression that contains data in a timestamp, datetime, date, epoch, or yyyymmdd format. The sql_start and sql_end fields can be any of the following:

  • A reference to a raw timeframe from an existing dimension group of type: time
  • A reference to a dimension of type: date_raw
  • A SQL expression that is a timestamp, such as a reference to a SQL column that is a timestamp
  • A SQL expression that pulls a time from your database, using the appropriate expression for your dialect
  • A LookML field reference using the ::datetime or ::date field type reference

As an example, suppose you have a dimension named faa_event_date_raw that contains datetime information:

dimension: faa_event_date_raw { type: date_raw sql: ${TABLE}.event_date ;; }

You can create a dimension group of type: duration that calculates the amount of time that has passed since the FAA event date. To do this, you can use the faa_event_date_raw dimension as the start time for the calculation, and then for the end time of the calculation you can use your dialect’s SQL expression for the current time. This example is for a MySQL database:

dimension_group: since_event { type: duration intervals: [hour, day] sql_start: ${faa_event_date_raw} ;; sql_end: CURRENT_TIMESTAMP();; }

In the Explore UI, this would generate a dimension group called Duration Since Event, with individual dimensions called Hours Since Event and Days Since Event.

Referencing intervals from another LookML field

To reference an interval value in a dimension_group of type: duration, use the syntax ${interval_fieldname}, using the plural version of the interval value. For example, in the following LookML example, the average_days_since_event measure uses ${days_since_event} to reference the day interval in the since_event dimension group:

dimension_group: since_event { type: duration intervals: [hour, day, week, month, quarter, year] sql_start: ${faa_event_date_raw} ;; sql_end: CURRENT_TIMESTAMP();; } measure: average_days_since_event { type: average sql: ${days_since_event} ;; }

Using LookML field type references with duration fields

To create a custom duration field, you can specify a ::date or ::datetime reference type for the dimensions referenced in the sql_start and sql_end parameters of a dimension group of type: duration. The view_name.field_name::type syntax, described on the Incorporating SQL and referring to LookML objects documentation page, lets you create a ::date or ::datetime version of a field without casting the references to those dimensions to strings.

For example, suppose you have a created dimension group of type: time with timeframes of time, date, week, month, and raw, defined as follows:

dimension_group: created { type: time timeframes: [time, date, week, month, raw] sql: ${TABLE}.created_at ;; }

Using the dimensions created_month and created_time, you can create a dimension group of type: duration that calculates the amount of time between a date from the created_date field and the first day of the month in which that date occurred, measured in weeks, days, and hours:

dimension_group: since_first_of_month { type: duration intervals: [week, day, hour] sql_start: ${created_month::datetime} ;; sql_end: ${created_time::datetime} ;; }

In the Explore UI, this creates a dimension group called Duration Since First of Month, with individual dimensions Weeks Since First of Month, Days Since First of Month, and Hours Since First of Month. Specifying the ::datetime reference type for the fields referenced in the sql_start and sql_end parameters allows the created_month and created_time dimensions to be treated as timestamps in the generated SQL.

As an example, suppose a user selects the Created Date and Days Since First of Month dimensions from the field picker. If one of the values returned for Created Date is 2019-03-10, then the value returned for Days Since First of Month will be 9 days.

Time type

type: time is used in conjunction with a dimension_group and the timeframes parameter to create a set of time-based dimensions. For example, you could easily create a date, week, and month dimension based on a single timestamp column.

The form of a dimension group of type: time is:

dimension_group: dimension_group_name { type: time timeframes: [timeframe, timeframe, …] # valid timeframes described below sql: SQL expression ;; # often this is a single database column datatype: epoch| timestamp | datetime | date | yyyymmdd # defaults to datetime convert_tz: yes | no # defaults to yes }

For dimension groups of type: time:

  • The timeframes parameter is optional but is rarely skipped. It specifies one or more timeframes that should be generated by the dimension group. If timeframes is not included every timeframe option will be added to the dimension group. The possible choices are listed in the Timeframe options section on this page.

  • The sql parameter for type: time dimension groups can take any valid SQL expression that contains data in a timestamp, datetime, date, epoch, or yyyymmdd format.

  • The datatype parameter is optional. If your dimension group is not based on a datetime, you may specify an epoch, timestamp, date, or yyyymmdd format instead. It is described in greater detail in the Specifying the database datatype section on this page.

  • The convert_tz parameter is optional and lets you prevent automatic time zone conversion. It is described in greater detail in the Time zone conversions and convert_tz section on this page.

Although they are not listed here, many of the field-level parameters can be used with dimension groups as well.

As an example, suppose you had a column named created_at that contained datetime information. You want to create a date, week, and month dimension based on this datetime. You could use:

dimension_group: created { type: time timeframes: [date, week, month] sql: ${TABLE}.created_at ;; }

In the Explore UI, this would generate three dimensions with the names Created Date, Created Week, and Created Month. Note how the dimension_group name is combined with the timeframes to generate the dimension names.

Timeframe options

The timeframes parameter is supported only for dimension groups of type: time. For dimension groups of type: duration, use the intervals parameter instead.

The timeframes parameter tells the dimension group which dimensions it should produce. The options are:

Special Timeframes

Timeframe Description Example Output
raw The raw value from your database, without casting or time zone conversion. raw is accessible only within LookML and will not show up on the Explore page. The raw timeframe returns a timestamp, unlike most other timeframes that return a formatted string. It is primarily used for performing date operations on a field. 2014-09-03
17:15:00 +0000
yesno A yesno dimension, returning “Yes” if the datetime has a value, otherwise “No”. Unlike other timeframes, when you refer to a yesno timeframe dimension from another field, do not include the timeframe in the reference. For example, to refer to a yesno timeframe in the dimension_group: created, use the syntax ${created}, not ${created_yesno}. Yes

Time Timeframes

Timeframe Description Example Output
time Datetime of the underlying field (some SQL dialects show as much precision as your database contains, while others show only to the second) 2014-09-03 17:15:00
time_of_day Time of day 17:15
hour Datetime truncated to the nearest hour 2014-09-03 17
hour_of_day Integer hour of day of the underlying field 17
hourX Splits each day into intervals with the specified number of hours. For an explanation, see below. See Below
minute Datetime truncated to the nearest minute 2014-09-03 17:15
minuteX Splits each hour into intervals with the specified number of minutes. For an explanation, see below. See Below
second Datetime truncated to the nearest second 2014-09-03 17:15:00
millisecond Datetime truncated to the nearest millisecond (see the Dialect support for milliseconds and microseconds section on this page for information on dialect support). 2014-09-03 17:15:00.000
millisecondX Splits each second into intervals with the specified number of milliseconds (see the Dialect support for milliseconds and microseconds section on this page for information on dialect support). For an explanation, see below. See Below
microsecond Datetime truncated to the nearest microsecond (see the Dialect support for milliseconds and microseconds section on this page for information on dialect support). 2014-09-03 17:15:00.000000

Date Timeframes

Timeframe Description Example Output
date Date of the underlying field 2017-09-03

Week Timeframes

Timeframe Description Example Output
week Date of the week starting on a Monday of the underlying datetime 2017-09-01
day_of_week Day of week alone Wednesday
day_of_week_index Day of week index (0 = Monday, 6 = Sunday) 2

Month Timeframes

Timeframe Description Example Output
month Year and month of the underlying datetime 2014-09
month_num Integer number of the month of the underlying datetime 9
fiscal_month_num Integer number of the fiscal month of the underlying datetime 6
month_name Name of the month September
day_of_month Day of month 3

To use the fiscal_month_num timeframes, the fiscal_month_offset parameter must be set in the model.

Quarter Timeframes

Timeframe Description Example Output
quarter Year and quarter of the underlying datetime 2017-Q3
fiscal_quarter Fiscal year and quarter of the underlying datetime 2017-Q3
quarter_of_year Quarter of the year preceded by a “Q” Q3
fiscal_quarter_of_year Fiscal quarter of the year preceded by a “Q” Q3

To use the fiscal_quarter and fiscal_quarter_of_year timeframes, the fiscal_month_offset parameter must be set in the model.

Year Timeframes

Timeframe Description Example Output
year Integer year of the underlying datetime 2017
fiscal_year Integer fiscal year of the underlying datetime FY2017
day_of_year Day of year 143
week_of_year Week of the year as a number 17

To use the fiscal_year timeframe, the fiscal_month_offset parameter must be set in the model.

Using hourX

In hourX the X is replaced with 2, 3, 4, 6, 8, or 12.

This will split up each day into intervals with the specified number of hours. For example, hour6 will split each day into 6 hour segments, which will appear as follows:

  • 2014-09-01 00:00:00
  • 2014-09-01 06:00:00
  • 2014-09-01 12:00:00
  • 2014-09-01 18:00:00

To give an example, a row with a time of 2014-09-01 08:03:17 would have a hour6 of 2014-09-01 06:00:00.

Using minuteX

In minuteX the X is replaced with 2, 3, 4, 5, 6, 10, 12, 15, 20, or 30.

This will split up each hour into intervals with the specified number of minutes. For example, minute15 will split each hour into 15 minute segments, which will appear as follows:

  • 2014-09-01 01:00:00
  • 2014-09-01 01:15:00
  • 2014-09-01 01:30:00
  • 2014-09-01 01:45:00

To give an example, a row with a time of 2014-09-01 01:17:35 would have a minute15 of 2014-09-01 01:15:00.

Using millisecondX

In millisecondX the X is replaced with 2, 4, 5, 8, 10, 20, 25, 40, 50, 100, 125, 200, 250, or 500.

This will split up each second into intervals with the specified number of milliseconds. For example, millisecond250 will split each second into 250 millisecond segments, which will appear as follows:

  • 2014-09-01 01:00:00.000
  • 2014-09-01 01:00:00.250
  • 2014-09-01 01:00:00.500
  • 2014-09-01 01:00:00.750

To give an example, a row with a time of 2014-09-01 01:00:00.333 would have a millisecond250 of 2014-09-01 01:00:00.250.

Time zone conversions and convert_tz

In general, time computations (differences, durations, etc.) only work correctly when you operate on time values that are all converted to the same time zone, so it is important to keep time zones in mind when writing LookML.

Looker has various time zone settings that convert time-based data between different time zones. Looker does time zone conversion by default. The convert_tz parameter is supported for dimension groups of type: time. If you do not want Looker to perform a time zone conversion for a particular dimension or dimension group, you can use the convert_tz parameter described on the convert_tz parameter documentation page.

Dialect support for milliseconds and microseconds

Looker supports timeframe precision to microseconds; however, some databases support precision only to the second. If a database encounters a timeframe more precise than it can support, it will round up to seconds.

In Looker 22.6, the following dialects support milliseconds:

In Looker 22.6, the following dialects support microseconds:

Specifying the database datatype

The datatype parameter lets you specify the type of time data in your database table that you are supplying to the dimension group, which can increase query performance.

For dimension groups of type: time, the datatype parameter applies to the sql parameter of the dimension group.

For dimension groups of type: duration, the datatype parameter applies to both the sql_start and sql_end parameters, so be sure the sql_start and sql_end are both of the specified data type.

The datatype parameter accepts the following values:

  • epoch: A SQL epoch field (i.e., an integer representing the number of seconds from the Unix epoch).
  • date: A SQL date field (i.e., one that does not contain time of day information).
  • datetime: A SQL datetime field.
  • timestamp: A SQL timestamp field.
  • yyyymmdd: A SQL field that contains an integer that represents a date of the form YYYYMMDD.

The default value for datatype is timestamp.

Examples

Suppose you had a column named created_at that contained datetime information. You want to create a date, week, and month dimension based on this datetime. You could use:

dimension_group: created { type: time timeframes: [date, week, month] sql: ${TABLE}.created_at ;; }

In the Explore UI, this would generate three dimensions with the names Created Date, Created Week, and Created Month. Note how the dimension_group name is combined with the timeframes to generate the dimension names.

Things to consider

Dimension groups must be referenced by their individual dimensions

Because a dimension group represents a group of dimensions, instead of just one dimension, you cannot refer to it directly in LookML. Instead, you’ll need to refer to the dimensions it creates.

For example, consider this dimension group:

dimension_group: created { type: time timeframes: [date, week, month] sql: ${TABLE}.created_at ;; }

To refer to one of these dimensions in another LookML field, use the reference ${created_date}, ${created_week}, or {$created_month}. If you try to use just ${created}, Looker will not know which timeframe you are referring to and an error will result.

For this same reason, you should not use the primary_key parameter on a dimension group if you specify more than one timeframe.

Chat Team Tip: We are frequently asked about the validation error that occurs if using primary_key on a dimension_group with more than one timeframe. For more information, check out this Community topic.

Timestamp data that includes time zone information

Some database dialects have timestamp options that include time zone information. This lets you store timestamp data in a single field that may have multiple time zones. One row of data might be stored in UTC, another row in Eastern time. As an example, see the Snowflake TIMESTAMP_LTZ, TIMESTAMP_NTZ, TIMESTAMP_TZ timestamp documentation for information about the Snowflake dialect timestamp options.

In this case, when Looker performs time zone conversions, errors can occur. To avoid this, in the sql parameter of the dimension, you should explicitly cast the timestamp data to a timestamp type that does not do time zone conversion. For example, in the Snowflake dialect, you could use the TO_TIMESTAMP function to cast the timestamp data.

It is possible to create individual time or duration dimensions

It is possible to create one dimension for each individual timeframe or duration you want to include, instead of generating all of them in a single dimension_group. You can generally avoid creating individual dimensions, unless you want to change Looker’s timeframe naming convention, or you have already pre-calculated time columns in your database. For more information, see the Dimension, filter, and parameter types documentation page.

You can change the first day of the week

By default, weeks in Looker start on Monday. You can change this by using the week_start_day parameter at the model level.

Just keep in mind that week_start_day does not work with the week_of_year timeframe because that timeframe is based on the ISO standard, which uses Monday weeks.

Custom filters and custom fields do not support all timeframes

Currently, the timeframes day_of_week, fiscal_quarter_of_year, millisecond, millisecondX, microsecond, month_name, quarter_of_year, and time_of_day are not supported in custom filters or custom fields.

Month, quarter, and year intervals only count complete periods

The month interval in a duration dimension group only considers a month to have passed if the ending day is greater than or equal to the starting day. For example:

  • The difference in months between September 26 and October 25 of the same year is 0.
  • The difference in months between September 26 and October 26 of the same year is 1.

The quarter and year intervals follow the same logic.

Related articles