# CSV, JSON and SQLITE3

Summary:

- sqlite3 is the most widely deployed database engine in the world
- JSON is useful as an intermediate format between CSV and sqlite3:
  - more API-friendly than CSV
  - more git- and diff-friendly than binary formats (like sqlite3)
  - can hold metadata as well as data
  - can be converted to other formats (e.g. parquet) with other existing
    utilities
  - with an appropriate schema, can be efficiently processed as a stream
- Many common approaches to converting CSV or sqlite3 to JSON use a JSON Schema
  with unnecessarily limits to utility, extensibility and/or performance
  efficiency. With a few tweaks to the target schema, however, you can have your
  cake and eat it
- [`zsv`](../README.md) provides high-performance conversion between CSV (or
  similar), JSON and sqlite3 (and for running SQL queries against CSV)

## Background

Each of these formats, commonly used for tabular data, has different strengths
and weaknesses:

- CSV is easy to use in any text editor or common apps such as Excel or Google
  Sheets, but lacks schema, data types or indexing

- JSON is easy to use for API inter-communication and REST APIs, editable with
  any text editor, supports primitive data types; but, lacks native indexing or
  schema (though JSON Schema can be overlaid for validation and documentation)
  and is difficult for non-technical users to use or analyze

- sqlite3 supports schema, indexes and other SQL operations, but requires
  sqlite3 and knowledge of SQL to use, and file format is binary which is not
  human-readable and is difficult to version-track either metadata or data

## Benefits of converting between formats

- easily stream CSV or DB data through APIs and back to CSV / DB

- use standard version control on sqlite3 files, covering metadata and data, by
  converting to JSON and tracking that instead of the binary DB file

- Using DB over CSV offers specific benefits not available in other formats such
  as function-based indexes and much more

## Using `zsv` to work with data in your format of choice

`zsv` provides high-performance conversion to and from various formats
including:

- CSV (or TSV or pipe- or other- delimited)
- JSON (as arrays, objects, or [database-friendly table schema](db.schema.json))
- sqlite3

To install `zsv`, see <https://github.com/liquidaty/zsv/blob/main/BUILD.md>.

`zsv`'s `2json`, `2db` and `jq` commands provide conversion between delimited,
json and sqlite3 formats. A streamlined JSON schema is used to retain metadata
when converting between JSON and sqlite3 formats, using the
[zsv](https://github.com/liquidaty/zsv), [jq](https://github.com/stedolan/jq)
and [sqlite3](https://www.sqlite.org) libraries.

### Getting some sample data

Let's get some sample data to work with:

```
curl -LO https://raw.githubusercontent.com/datasets/world-cities/master/data/world-cities.csv
```

### CSV to JSON

There are multiple JSON Schemas that can be used to represent CSV data. The most common
ones, however, have significant drawbacks. `zsv` supports conversion to common / limited
formats as well as more versatile schemas described below.

#### The common (and not very useful) way

The most common CSV to JSON conversion approaches, referred to herein as the
"plain-array" and "plain-objects" approaches, have limited utility:

```shell
zsv 2json world-cities.csv --no-header < world-cities.csv # output as arrays
zsv 2json world-cities.csv --object < world-cities.csv    # output as objects
```

The problem with the first format is that it treats the header row as a data row
and lacks support for additional metadata such as data type.

The second format has the same weakness and furthermore takes up significantly
more space, though it has the advantage of each row being independently
human-readable without without having to look at the header information to know
which data corresponds to which column. That said, this advantage has limited
benefit only useful, in the best case, for debugging purposes. To quantify the
difference in data volume, using the first 10k rows of worldcitiespop.csv, the
`plain-object` format is 78% larger or 117% larger for pretty-printed or compact
JSON, respectively.

#### Adding column metadata

A better approach is to replace each header name with an object, to provide a
means to store metadata separately from data. One approach is to simply allow
the first row to use an object in lieu of a scalar galue, in what we refer to
herein as a [row-based schema](csv.schema.json):

```shell
# output as an array of rows, where the first row is a list of column header objects
zsv 2json < world-cities.csv > world-cities.json

# back to CSV
zsv jq '[.[0]|[.[]|.name]] + .[1:]|.[]' --csv < world-cities.json

# or
zsv jq '.[]|[.[]|(if (.|type) == "object" then .name else . end)]' --csv < world-cities.json
```

Note that the above `zsv jq` filter is essentially the same as what you would
use with `jq` in conjunction with `@csv` and `-r`, but the csv output from `zsv
jq ... --csv` is a bit more compact due to the omission of unnecessary
surrounding quotes

This approach has the same overall structure as the `plain-array` method, but
extends each cell's schema to allow for an object that can hold metadata. This
provides significantly more flexibility, but still has a significant drawback:
it does not allow for table-level metadata.

### (CSV to) JSON to Sqlite

#### Adding table metadata

To support both table and column metadata, `zsv` uses a [database-friendly
approach](db.schema.json) this supports table metadata (such as table name and
indexes) through a tuple (ordered list) containing two elements (metadata
followed by data). At the table level, this approach could also be merged with
the aforementioned [row-based schema](csv.schema.json) to support row-level or
cell-level metadata, to represent spreadsheet data such as cell-level
formatting.

The `zsv` conversion implementations as of this writing are limited; future
improvements under consideration include:

- automatic determination / suggestion of column data type
- conversion/coercion of text to the specified data type for the corresponding
  column
- load to other database back-end e.g. mssql or redshift

Future enhancements to both the schema and the conversion implementation under
consideration include support for other table-level features such as
validations/constraints, other database object types such as views, and
whole-database conversion.

```shell
# Convert CSV to JSON
# Additionally, specify a non-unique index on Population and a unique index on the combination of City, Country and Region
zsv 2json --database --index "ix1 on name, country" --unique-index "ix2 on geonameid" --db-table "world-cities" < world-cities.csv > world-cities.db.json

# Convert the resulting JSON to a sqlite3 database
zsv 2db -o world-cities.db < world-cities.db.json

# or do the above 2 steps in a compound one-liner. use --overwrite to replace the db we just created
zsv 2json --database --index "ix1 on name, country" --unique-index "ix2 on geonameid" --db-table "world-cities" < world-cities.csv | zsv 2db -o world-cities.db --overwrite

# query with sqlite3
sqlite3 world-cities.db ".headers on" ".mode csv" "select * from [world-cities] limit 10"

##### output from query #####
###  name,country,subcountry,geonameid
###  "les Escaldes",Andorra,Escaldes-Engordany,3040051
###  "Andorra la Vella",Andorra,"Andorra la Vella",3041563
###  "Umm al Qaywayn","United Arab Emirates","Umm al Qaywayn",290594
###  "Ras al-Khaimah","United Arab Emirates","Raʼs al Khaymah",291074
###  "Khawr Fakkān","United Arab Emirates","Ash Shāriqah",291696
###  Dubai,"United Arab Emirates",Dubai,292223
###  "Dibba Al-Fujairah","United Arab Emirates","Al Fujayrah",292231
###  "Dibba Al-Hisn","United Arab Emirates","Al Fujayrah",292239
###  Sharjah,"United Arab Emirates","Ash Shāriqah",292672
###  "Ar Ruways","United Arab Emirates","Abu Dhabi",292688

# JSON back to CSV
zsv jq '[.[0]|.columns|[.[]|.name]] + .[1]|.[]' --csv < world-cities.db.json

## or
zsv jq '.[]|if (.|type) == "object" then (.columns|[.[]|.name]) else . end|.[]' --csv < world-cities.db.json
```

### Sqlite to JSON

Now let's go the other way:

```shell
# sqlite3 to JSON
zsv 2json --from-db world-cities.db > world-cities.from-db.json

# verify it's the same as what we generated above
cmp world-cities.from-db.json world-cities.db.json && echo "It's the same"'!' || echo "Uh oh"
```

## Questions

### Why the bundled / "swiss army knife" approach?

An alternative approach, which is more consistent with the unix philosophy
whereby "each utility does one and only one thing", would be leave anything
related to sqlite3 and JSON to `sqlite3`, `jq`, respectively, or similar
standalone utilities.

However, the unix philosophy also assumed that a full toolset would always be
available, whereas we cannot assume that anyone already has `jq` or `sqlite3`
installed. It is true that we could have distributed them with this package but
then we introduce potential conflict where they were already installed.
Furthermore, there are other benefits to bundling:

- tasks that involve multiple utilities can be further simplified and
  modularized For example, CSV->DB conversion can be further broken down into
  CSV->JSON->DB, and the middle JSON form can be independently manipulated. This
  additional modularization / granularity provides greater optionality with
  regard to flexibility and control of how data is processed

- deployment is simplified (there is only one executable), and the total
  executable code size is compact even when fully statically compiled. this is
  convenient in environments where dynamically-linked code can be challenging,
  such as WebAssembly

The main drawback of the bundled approach is that if a single command needs to
be updated, the entire executable must be updated. In most cases, our experience
is that the above benefits are worth this cost. However, one need not choose one
over the other: the `zsv` source also supports unbundled binary builds (though
the unbundled binaries are not part of the officially supported packages).

Note that `zsv` does not attempt to replace `jq` or `sqlite3` and only
integrates a portion of their features that is necessary for basic conversion,
testing and other common pipeline operations.

### For each of these JSON schemas, why is top-level construct is an array and not an object

At some point while reading this, you may have wondered: why is the JSON schema
an array instead of an object? Why not something like:

```json
{
  metadata: ... # columns, indexes, whatever
  data: ...
}
```

Indeed, to programmers familiar with data structures, this would seem like a
logical approach.

One of the central design principles that `zsv` follows is to support
stream-based processing whenever possible, in order to maximize performance
efficiency. To do that, serialized data must always locate metadata before data.

Unfortunately, JSON does not support ordered properties within an object. As a
result, if the top-level container was an object, there would be no guarantee
that when serialized, metadata would always be written (or read, in the case of
the recipient) before data. The only way to enforce order in JSON is to use a
list.

### For the database-compatible JSON schema, why use a 2-element tuple?

An alternative to the 2-element tuple would be to simply use a list such as is
used in the [row-based schema](csv.schema.json). The primary reason to prefer
the tuple approach is *optionality*. Using the tuple approach still offers the
option to support everything the row-based schema offers, while furthermore
offering the benefit of a container that is unambiguously and solely dedicated
to metadata.

### How can this approach be extended to databases or database services (vs a single table at a time)?

To extend this approach to an entire database, there are several objectives to
consider:

1. coverage: all metadata (and data) must be covered
2. modularity: metadata must be separable not from data, but also within its own
   categories (e.g. tables vs indexes)
3. performance: metadata and data must be serializable in a form that can be
   efficiently unserialized

The [database-friendly table schema](db.schema.json) is compatible with all of
these objectives.

Table and column metadata is already covered, so the only remaining levels of
metadata to address are database and database-service. These could be handled
through a top-level object with properties such as "tables", "views" etc.

However, for serialization, it may be advantageous to output all metadata for
all tables (and other objects) before any data. In addition, there may be
reasons to want to specify the order in which metadata objects or table data is
populated. As discussed above for table objects, this requires ordering which in
turn requires a list rather than an object.

So putting them all together, a database-wide schema could be structured as a
(metadata, data) tuple as follows:

```text
# first, define a metadata object with a discriminator
{
  type: table|view|...
  object: [meta, data]    # if type = table, the object will be in [database-friendly table schema](db.schema.json)
}

# now, our database tuple:
[
  # metadata comes first
  [
    object1, # only the first (metadata) element of the tuple should be populated
    object2,
    ...
  ],
  
  # table data comes next, in an ordered list
  [
    object1, # the first element should contain the object name, and the second populated with data
    object2,
    ...
  ]
]
```

A similar approach could be used for a collection of databases. The above is not
meant to be a comprehensive final design, but hopefully conveys the basic idea
that the [database-friendly table schema](db.schema.json) will not require
breaking change in order to leverage for extended purposes

### Can you extend the `zsv 2db` command and/or related JSON schema to support the sqlite3 feature XYZ

If it's supported in the sqlite3 library, the answer is generally yes. Please
[open an issue](https://github.com/liquidaty/zsv/issues/new/choose) with your
request.
