---
title: "Getting Started with qryflow"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{Getting Started with qryflow}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r, include = FALSE}
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
```

```{r}
library(qryflow)
```

# What is `qryflow`?

The `qryflow` package lets you tag sections of SQL, called chunks, so that R knows how to execute each part independently. Tagging your SQL lets you:

- Keep multiple SQL statements in the same file.

- Control how SQL is executed (e.g., `DBI::dbGetQuery()`, `dbExecute()`)

- Attach names to results, so they're returned as named R objects

- Pass metadata that can be used later in R workflows

In short: You can define and run **multi-step SQL workflows** with one function call, and get your results back as a structured R object.

This vignette covers:

- How to tag SQL chunks

- The different types of tags supported

- How `qryflow` defines and handles a chunk

# Basic usage

The main function is `qryflow`, which accepts SQL tagged with special comments and a connection to DBI-compliant database. Note, the SQL can be a character vector, like in the example below, or a filepath to a file that contains SQL.

```{r example}
library(qryflow)

# Connection to In-Memory DB with table populated from mtcars
con <- example_db_connect(mtcars)

sql <- "
-- @exec: drop_cyl_6
DROP TABLE IF EXISTS cyl_6;

-- @exec: prep_cyl_6
CREATE TABLE cyl_6 AS
SELECT *
FROM mtcars
WHERE cyl = 6;

-- @query: df_mtcars
SELECT *
FROM mtcars;

-- @query: df_cyl_6
SELECT *
FROM cyl_6;
"

# Pass tagged SQL to `qryflow`
results <- qryflow(sql, con)

# Access the results from the chunk named `df_cyl_6`
head(results$df_cyl_6)
```

When you run `qryflow()`, here's what happens:

1. Your SQL script is split into chunks using tag lines like `-- @query: df_mtcars`.

2. Each chunk is assigned a type (e.g., `query` or `exec`)

3. Chunks are executed in order, and query results are returned as named objects

# Simplify = TRUE

By default, `qryflow()` will return a single data.frame (as opposed to a named list of results) if there is only one chunk and the argument `simplify=TRUE`. For example:

```{r}
library(qryflow)

# Connection to In-Memory DB with table populated from mtcars
con <- example_db_connect(mtcars)

sql <- "
-- @query: df_mtcars
SELECT *
FROM mtcars;
"

# Pass tagged SQL to `qryflow`
results <- qryflow(sql, con)

# Access the results from the chunk named `df_cyl_6`
# results$df_cyl_6
head(results)
```

This design choice is to facilitate easy interactive use and is a common use-case. Because `qryflow()` might return a named `list` or a single `data.frame` depending on the input, `qryflow` exports other functions so users can prioritize reliability in return objects. See the help pages for `?qryflow_run()` and `?qryflow_results()`. Or check out the Advanced Usage vignette.

# Tagging syntax

To tag a specific chunk of SQL, use the following format: `-- @key: value`.

For example, to indicate that the following chunk of SQL is a `query`, meaning we expect it to return a `data.frame` by calling `DBI::dbGetQuery`, we precede the statment with a special comment:

```sql
-- @query: my_data_frame
SELECT *
FROM TBL
WHERE COLUMN = 'VALUE'
```

Breaking down the tag into it's component parts:

1. The tag begins with two dashes (`--`). This indicates a single line comment in SQL.

2. Next, we use the `@` symbol to indicate the start of a tag, followed by the tag type. Currently, `qryflow` formally supports four tags: `type`, `name`, `query`, and `exec`. We follow the tag type with a colon (`:`).

3. Next comes a value, depending on the type of tag. For the `@type:` tag, this will indicate the "type" of SQL chunk. For the `name`, `query`, and `exec` tags, the value indicates the custom name of the SQL chunk.

# Tags and Aliases

Each SQL chunk must be tagged so `qryflow` knows how to handle it. Tags use SQL-style comments (`--`) and follow the format:

```sql
-- @<tag>: <value>
```

## Common Tags

- `@type`	Specifies execution type (`-- @type: query`)

- `@name`	Assigns a name to the chunk’s result (`-- @name: df_users`)

- `@query` Executes SQL with `DBI::dbGetQuery()` and names the result (`-- @query: df_users`)

- `@exec`	Executes SQL with `DBI::dbExecute()` (`-- @exec: drop_table`)


`@query` and `@exec` are aliases for setting both `@type` and `@name` in one line.

**Aliased form (preferred):**

```sql
-- @query: df_mtcars
SELECT *
FROM mtcars;
```
**Explicit form (equivalent):**

```sql
-- @type: query
-- @name: df_mtcars
SELECT *
FROM mtcars;
```

## Custom Tags

You can include additional tags (e.g., `-- @source: users`) to carry metadata into your R workflow. Only one execution-related tag is required per chunk (@query, @exec, or @type).

# Defining a Chunk

In `qryflow`, a Chunk is a logically grouped section of SQL code, representing a single executable unit within a larger multi-step SQL workflow.

The SQL script is split into multiple chunks using specially formatted tag lines (SQL comments beginning with tags like -- @query: or -- @exec:). It scans through the SQL lines and splits the script at these tagged lines, grouping all subsequent SQL lines until the next tag or the end of the script.

## How Splitting Works

- Tagged lines act as markers that start a new chunk.

- All lines following a tag line belong to that chunk until another tag line starts the next chunk.

- If the script starts without any tags, the entire script is treated as one single chunk.

- If there’s only one tag somewhere in the script, the script is split into two chunks:

    - Everything before the tag becomes the first chunk (even if untagged).

    - The tagged line and everything after it become the second chunk.
    
## Examples

**Example 1 - Script with no tags**

```sql
CREATE TABLE cyl_6 AS
SELECT *
FROM mtcars
WHERE cyl = 6;
```
Result - The entire script is one chunk containing all lines.

- Why? Without tags, `qryflow` treats the whole script as a single step.

**Example 2 - Script with one tag at the start**

```sql
-- @query: get_6cyl
SELECT *
FROM mtcars
WHERE cyl = 6;
```
Result - One chunk starting at the tag, containing the rest of the script.

Because the tag is at line 1, the chunk starts there and continues to the end.

**Example 3 - Script with one tag in the middle**

```sql
SELECT *
FROM mtcars
WHERE cyl = 6;

-- @query: df_mtcars
SELECT *
FROM mtcars;
```

Result - Two chunks:

- Chunk 1: lines before the tag (untagged SQL).

- Chunk 2: from the tag line to the end.

This preserves any pre-tag SQL as a separate chunk.

**Example 4 - Script with multiple tags**

```sql
-- @exec: drop_cyl_6
DROP TABLE IF EXISTS cyl_6;

-- @exec: prep_cyl_6
CREATE TABLE cyl_6 AS
SELECT *
FROM mtcars
WHERE cyl = 6;

-- @query: df_mtcars
SELECT *
FROM mtcars;

-- @query: df_cyl_6
SELECT *
FROM cyl_6
```

- Result - Three chunks, each starting at its respective tag line.

- Each chunk is parsed and executed independently in sequence.

# Type Identification

Before a chunk is parsed or executed, `qryflow` determines its type using the following rules:

1. If a chunk includes an explicit `-- @type:` tag, that value is used as the chunk type.

2. If there is no `@type:` tag, `qryflow` checks for any other tag (`@query:`, `@exec:`, etc.) that matches a registered type. The first match found is used as the type.

3. If no recognized tag is found, the type defaults to the value of `getOption("qryflow.default.type", "query")`.

Note that formally, "query" is the default type of `qryflow` if the option is not overridden.

```{r, echo=FALSE, include=FALSE}
DBI::dbDisconnect(con)
```