R: Bind values to a parameterized/prepared statement

dbBind {DBI} | R Documentation

## Bind values to a parameterized/prepared statement

### Description

For parametrized or prepared statements, the `dbSendQuery()` and `dbSendStatement()` functions can be called with statements that contain placeholders for values. The `dbBind()` function binds these placeholders to actual values, and is intended to be called on the result set before calling `dbFetch()` or `dbGetRowsAffected()`.

### Usage

    dbBind(res, params, ...)

### Arguments

`res` | An object inheriting from DBIResult.
`params` | A list of bindings, named or unnamed.
`...` | Other arguments passed on to methods.

### Details

DBI supports parametrized (or prepared) queries and statements via the `dbBind()` generic. Parametrized queries are different from normal queries in that they allow an arbitrary number of placeholders, which are later substituted by actual values. Parametrized queries (and statements) serve two purposes:

- The same query can be executed more than once with different values. The DBMS may cache intermediate information for the query, such as the execution plan, and execute it faster.

- Separation of query syntax and parameters protects against SQL injection.

The placeholder format is currently not specified by DBI; in the future, a uniform placeholder syntax may be supported. Consult the backend documentation for the supported formats. For automated testing, backend authors specify the placeholder syntax with the `placeholder_pattern` tweak. Known examples are:

- `?` (positional matching in order of appearance) in RMySQL and RSQLite

- `$1` (positional matching by index) in RPostgres and RSQLite

- `:name` and `$name` (named matching) in RSQLite

### Value

`dbBind()` returns the result set, invisibly, for queries issued by `dbSendQuery()` and also for data manipulation statements issued by `dbSendStatement()`. Calling `dbBind()` for a query without parameters raises an error. Binding too many or not enough values, or parameters with wrong names or unequal length, also raises an error. If the placeholders in the query are named, all parameter values must have names (which must not be empty or `NA`), and vice versa, otherwise an error is raised. The behavior for mixing placeholders of different types (in particular mixing positional and named placeholders) is not specified.

Calling `dbBind()` on a result set already cleared by `dbClearResult()` also raises an error.

### Specification

DBI clients execute parametrized statements as follows:

1. Call `dbSendQuery()` or `dbSendStatement()` with a query or statement that contains placeholders, store the returned DBIResult object in a variable. Mixing placeholders (in particular, named and unnamed ones) is not recommended. It is good practice to register a call to `dbClearResult()` via `on.exit()` right after calling `dbSendQuery()` or `dbSendStatement()` (see the last enumeration item). Until `dbBind()` has been called, the returned result set object has the following behavior:

   - `dbFetch()` raises an error (for `dbSendQuery()`)

   - `dbGetRowCount()` returns zero (for `dbSendQuery()`)

   - `dbGetRowsAffected()` returns an integer `NA` (for `dbSendStatement()`)

   - `dbIsValid()` returns `TRUE`

   - `dbHasCompleted()` returns `FALSE`

2. Construct a list with parameters that specify actual values for the placeholders. The list must be named or unnamed, depending on the kind of placeholders used. Named values are matched to named parameters, unnamed values are matched by position in the list of parameters. All elements in this list must have the same lengths and contain values supported by the backend; a data.frame is internally stored as such a list. The parameter list is passed to a call to `dbBind()` on the `DBIResult` object.

3. Retrieve the data or the number of affected rows from the `DBIResult` object.

   - For queries issued by `dbSendQuery()`, call `dbFetch()`.

   - For statements issued by `dbSendStatements()`, call `dbGetRowsAffected()`. (Execution begins immediately after the `dbBind()` call, the statement is processed entirely before the function returns.)

4. Repeat 2. and 3. as necessary.

5. Close the result set via `dbClearResult()`.

The elements of the `params` argument do not need to be scalars, vectors of arbitrary length (including length 0) are supported. For queries, calling `dbFetch()` binding such parameters returns concatenated results, equivalent to binding and fetching for each set of values and connecting via `rbind()`. For data manipulation statements, `dbGetRowsAffected()` returns the total number of rows affected if binding non-scalar parameters. `dbBind()` also accepts repeated calls on the same result set for both queries and data manipulation statements, even if no results are fetched between calls to `dbBind()`.

At least the following data types are accepted on input (including NA):

- integer

- numeric

- logical for Boolean values

- character

- factor (bound as character, with warning)

- Date

- POSIXct timestamps

- POSIXlt timestamps

- lists of raw for blobs (with `NULL` entries for SQL NULL values)

- objects of type blob::blob

### See Also

Other DBIResult generics: `DBIResult-class`, `dbClearResult`, `dbColumnInfo`, `dbFetch`, `dbGetInfo`, `dbGetRowCount`, `dbGetRowsAffected`, `dbGetStatement`, `dbHasCompleted`, `dbIsValid`, `dbQuoteIdentifier`, `dbQuoteLiteral`, `dbQuoteString`, `dbUnquoteIdentifier`

### Examples

    con <- dbConnect(RSQLite::SQLite(), ":memory:")

    dbWriteTable(con, "iris", iris)

    # Using the same query for different values
    iris_result <- dbSendQuery(con, "SELECT * FROM iris WHERE [Petal.Width] > ?")
    dbBind(iris_result, list(2.3))
    dbFetch(iris_result)
    dbBind(iris_result, list(3))
    dbFetch(iris_result)
    dbClearResult(iris_result)

    # Executing the same statement with different values at once
    iris_result <- dbSendStatement(con, "DELETE FROM iris WHERE [Species] = $species")
    dbBind(iris_result, list(species = c("setosa", "versicolor", "unknown")))
    dbGetRowsAffected(iris_result)
    dbClearResult(iris_result)

    nrow(dbReadTable(con, "iris"))

    dbDisconnect(con)

[Package DBI version 0.8 Index]