description: "Ingest and query Tab Separated Value data in 5 steps"
title: NYPD Complaint Data
---
Tab separated value, or TSV, files are common and may include field headings as the first line of the file. ClickHouse can ingest TSVs, and also can query TSVs without ingesting the files. This guide covers both of these cases. If you need to query or ingest CSV files, the same techniques work, simply substitute `TSV` with `CSV` in your format arguments.
While working through this guide you will:
- **Investigate**: Query the structure and content of the TSV file.
- **Determine the target ClickHouse schema**: Choose proper data types and map the existing data to those types.
- **Create a ClickHouse table**.
- **Preprocess and stream** the data to ClickHouse.
- **Run some queries** against ClickHouse.
The dataset used in this guide comes from the NYC Open Data team, and contains data about "all valid felony, misdemeanor, and violation crimes reported to the New York City Police Department (NYPD)". At the time of writing, the data file is 166MB, but it is updated regularly.
**Terms of use**: https://www1.nyc.gov/home/terms-of-use.page
## Prerequisites
- Download the dataset by visiting the [NYPD Complaint Data Current (Year To Date)](https://data.cityofnewyork.us/Public-Safety/NYPD-Complaint-Data-Current-Year-To-Date-/5uac-w243) page, clicking the Export button, and choosing **TSV for Excel**.
- Install [ClickHouse server and client](../../getting-started/install.md).
- [Launch](../../getting-started/install.md#launch) ClickHouse server, and connect with `clickhouse-client`
### A note about the commands described in this guide
There are two types of commands in this guide:
- Some of the commands are querying the TSV files, these are run at the command prompt.
- The rest of the commands are querying ClickHouse, and these are run in the `clickhouse-client` or Play UI.
:::note
The examples in this guide assume that you have saved the TSV file to `${HOME}/NYPD_Complaint_Data_Current__Year_To_Date_.tsv`, please adjust the commands if needed.
Most of the time the above command will let you know which fields in the input data are numeric, and which are strings, and which are tuples. This is not always the case. Because ClickHouse is routineley used with datasets containing billions of records there is a default number (100) of rows examined to [infer the schema](/docs/en/integrations/data-ingestion/data-formats/json.md#relying-on-schema-inference) in order to avoid parsing billions of rows to infer the schema. The response below may not match what you see, as the dataset is updated several times each year. Looking at the Data Dictionary you can see that CMPLNT_NUM is specified as text, and not numeric. By overriding the default of 100 rows for inference with the setting `SETTINGS input_format_max_rows_to_read_for_schema_inference=2000`
Note: as of version 22.5 the default is now 25,000 rows for inferring the schema, so only change the setting if you are on an older version or if you need more than 25,000 rows to be sampled.
:::
Run this command at your command prompt. You will be using `clickhouse-local` to query the data in the TSV file you downloaded.
At this point you should check that the columns in the TSV file match the names and types specified in the **Columns in this Dataset** section of the [dataset web page](https://data.cityofnewyork.us/Public-Safety/NYPD-Complaint-Data-Current-Year-To-Date-/5uac-w243). The data types are not very specific, all numeric fields are set to `Nullable(Float64)`, and all other fields are `Nullable(String)`. When you create a ClickHouse table to store the data you can specify more appropriate and performant types.
### Determine the proper schema
In order to figure out what types should be used for the fields it is necessary to know what the data looks like. For example, the field `JURISDICTION_CODE` is a numeric: should it be a `UInt8`, or an `Enum`, or is `Float64` appropriate?
The query response shows that the `JURISDICTION_CODE` fits well in a `UInt8`.
Similarly, look at some of the `String` fields and see if they are well suited to being `DateTime` or [`LowCardinality(String)`](../../sql-reference/data-types/lowcardinality.md) fields.
For example, the field `PARKS_NM` is described as "Name of NYC park, playground or greenspace of occurrence, if applicable (state parks are not included)". The names of parks in New York City may be a good candidate for a `LowCardinality(String)`:
The dataset in use at the time of writing has only a few hundred distinct parks and playgrounds in the `PARK_NM` column. This is a small number based on the [LowCardinality](../../sql-reference/data-types/lowcardinality.md#lowcardinality-dscr) recommendation to stay below 10,000 distinct strings in a `LowCardinality(String)` field.
### DateTime fields
Based on the **Columns in this Dataset** section of the [dataset web page](https://data.cityofnewyork.us/Public-Safety/NYPD-Complaint-Data-Current-Year-To-Date-/5uac-w243) there are date and time fields for the start and end of the reported event. Looking at the min and max of the `CMPLNT_FR_DT` and `CMPLT_TO_DT` gives an idea of whether or not the fields are always populated:
-`PARKS_NM` should be cast to `LowCardinality(String)`
-`CMPLNT_FR_DT` and `CMPLNT_FR_TM` are always populated (possibly with a default time of `00:00:00`)
-`CMPLNT_TO_DT` and `CMPLNT_TO_TM` may be empty
- Dates and times are stored in separate fields in the source
- Dates are `mm/dd/yyyy` format
- Times are `hh:mm:ss` format
- Dates and times can be concatenated into DateTime types
- There are some dates before January 1st 1970, which means we need a 64 bit DateTime
:::note
There are many more changes to be made to the types, they all can be determined by following the same investigation steps. Look at the number of distinct strings in a field, the min and max of the numerics, and make your decisions. The table schema that is given later in the guide has many low cardinality strings and unsigned integer fields and very few floating point numerics.
:::
## Concatenate the date and time fields
To concatenate the date and time fields `CMPLNT_FR_DT` and `CMPLNT_FR_TM` into a single `String` that can be cast to a `DateTime`, select the two fields joined by the concatenation operator: `CMPLNT_FR_DT || ' ' || CMPLNT_FR_TM`. The `CMPLNT_TO_DT` and `CMPLNT_TO_TM` fields are handled similarly.
## Convert the date and time String to a DateTime64 type
Earlier in the guide we discovered that there are dates in the TSV file before January 1st 1970, which means that we need a 64 bit DateTime type for the dates. The dates also need to be converted from `MM/DD/YYYY` to `YYYY/MM/DD` format. Both of these can be done with [`parseDateTime64BestEffort()`](../../sql-reference/functions/type-conversion-functions.md#parsedatetime64besteffort).
"WITH (CMPLNT_FR_DT || ' ' || CMPLNT_FR_TM) AS CMPLNT_START,
(CMPLNT_TO_DT || ' ' || CMPLNT_TO_TM) AS CMPLNT_END
select parseDateTime64BestEffort(CMPLNT_START) AS complaint_begin,
parseDateTime64BestEffortOrNull(CMPLNT_END) AS complaint_end
FROM file('${HOME}/NYPD_Complaint_Data_Current__Year_To_Date_.tsv', 'TSVWithNames')
ORDER BY complaint_begin ASC
LIMIT 25
FORMAT PrettyCompact"
```
Lines 2 and 3 above contain the concatenation from the previous step, and lines 4 and 5 above parse the strings into `DateTime64`. As the complaint end time is not guaranteed to exist `parseDateTime64BestEffortOrNull` is used.
The dates shown as `1925` above are from errors in the data. There are several records in the original data with dates in the years `1019` - `1022` that should be `2019` - `2022`. They are being stored as Jan 1st 1925 as that is the earliest date with a 64 bit DateTime.
:::
## Create a table
The decisions made above on the data types used for the columns are reflected in the table schema
below. We also need to decide on the `ORDER BY` and `PRIMARY KEY` used for the table. At least one
## Preprocess and Import Data {#preprocess-import-data}
We will use `clickhouse-local` tool for data preprocessing and `clickhouse-client` to upload it.
### `clickhouse-local` arguments used
:::tip
`table='input'` appears in the arguments to clickhouse-local below. clickhouse-local takes the provided input (`cat ${HOME}/NYPD_Complaint_Data_Current__Year_To_Date_.tsv`) and inserts the input into a table. By default the table is named `table`. In this guide the name of the table is set to `input` to make the data flow clearer. The final argument to clickhouse-local is a query that selects from the table (`FROM input`) which is then piped to `clickhouse-client` to populate the table `NYPD_Complaint`.
[A Practical Introduction to Sparse Primary Indexes in ClickHouse](/docs/en/guides/best-practices/sparse-primary-indexes.md) discusses the differences in ClickHouse indexing compared to traditional relational databases, how ClickHouse builds and uses a sparse primary index, and indexing best practices.
