R: Query Column Structure in ODBC Tables

sqlColumns {RODBC} | R Documentation

## Query Column Structure in ODBC Tables

### Description

Enquire about the column structure of tables on an ODBC database connection.

### Usage

    sqlColumns(channel, sqtable, errors = FALSE, as.is = TRUE,
               special = FALSE, catalog = NULL, schema = NULL,
               literal = FALSE)

    sqlPrimaryKeys(channel, sqtable, errors = FALSE, as.is = TRUE,
                   catalog = NULL, schema = NULL)

### Arguments

`channel` | connection object as returned by `odbcConnect`.
`sqtable` | character string: a database table (or view or similar) name accessible from the connected DSN. If wildcards are allowed (only for `sqlColumns(special=FALSE)`), results for all matching tables.
`errors` | logical: if true halt and display error, else return `-1`.
`as.is` | see `sqlGetResults`.
`special` | logical. If true, return only the column(s) needed to specify a row uniquely. Depending on the database, there might be none.
`catalog, schema` | `NULL` or character: additional information on where to locate the table: see `sqlTables` for driver-specific details. Wildcards may be supported in `schema` for `sqlColumns(special=FALSE)`.
`literal` | logical: wildcards may be interpreted in `schema` and `sqtable`: if so this may suppress such interpretation.

### Details

The argument `special = TRUE` to `sqlColumns` returns the column(s) needed to specify a row uniquely. This is intended to form the basis of an SQL `WHERE` clause for update queries (see `sqlUpdate`), and what (if anything) it does is DBMS-specific. On many DBMSs it will return the primary keys if present: on others it will return a pseudo-column such as ROWID (Oracle) or _ROWID_ (SQLite), either always (Oracle) or if there is no primary key.

Primary keys are implemented in some DBMSs and drivers. A table can have a single column designated as a primary key or, in some cases, multiple columns. Primary keys should not be nullable (that is, cannot contain missing values). They can be specified as part of a CREATE TABLE statement or added by a ALTER TABLE statement.

In principle specifying `catalog` should select an alternative database in MySQL or an attached database in SQLite, but neither works with current drivers.

If `sqtable` contains . and neither `catalog` nor `schema` is supplied, an attempt is made to interpret `qualifier.table` as table `table` in schema `qualifier` (and for MySQL ‘schema’ means ‘database’, but the current drivers fail to interpret `catalog=`, so this does not yet work). (This can be suppressed by opening the connection with `interpretDot = FALSE`.) This has been tested successfully on PostgreSQL, SQL Server, Oracle, DB2 and Mimer.

Whether wildcards are accepted for `sqtable` and `schema` in `sqlColumns(special = FALSE)` depends on the driver and may be changed by the value of `literal`. For example, the PostgreSQL driver tested allowed wildcards in `schema` only if `literal = FALSE` and never in `sqtable`, whereas two MySQL drivers both failed to match a database when `catalog` was supplied and always allowed wildcards in `sqtable` even if `literal = TRUE`.

### Value

A data frame on success. If no data is returned, either a zero-row data frame or an error. (For example, if there are no primary keys or special column(s) in this table an empty data frame is returned, but if primary keys are not supported by the ODBC driver or DBMS, an error code results.)

The column names are not constant across ODBC versions so the data should be accessed by column number.

For `sqlPrimaryKeys` and `sqlColumns(special=FALSE)` the first four columns give the catalog, schema, table and column names (where applicable). For `sqlPrimaryKeys` the next two columns are the column sequence number (starting with 1) and name of the primary key: drivers can define further columns. For `sqlColumns(special=FALSE)` there are 18 columnns: see https://docs.microsoft.com/en-us/sql/odbc/reference/syntax/sqlcolumns-function?view=sql-server-ver15. Those beyond the first 6 shown in the examples give the ‘ordinal position’ (column 17) and further characteristics of the column type: see `sqlTypeInfo`.

For the numeric values returned by `sqlColumns(special=TRUE)` see https://docs.microsoft.com/en-us/sql/odbc/reference/syntax/sqlspecialcolumns-function?view=sql-server-ver15: the scope should always be `2` (the session) since that is the scope requested in the call. For the `PSEUDO_COLUMN` column, the possible values are `0` (unknown), `1` (no) and `2` (yes).

### Author(s)

Michael Lapsley and Brian Ripley

### See Also

`odbcConnect`, `sqlQuery`, `sqlFetch`, `sqlSave`, `sqlTables`, `odbcGetInfo`

### Examples

    ## Not run: ## example results from MySQL
    > channel <- odbcConnect("test")
    > sqlDrop(channel, "USArrests", errors = FALSE) # precautionary
    > sqlSave(channel, USArrests, addPK = TRUE)
    > sqlColumns(channel, "USArrests")
      TABLE_CAT TABLE_SCHEM TABLE_NAME COLUMN_NAME DATA_TYPE TYPE_NAME
    1    ripley        <NA>  USArrests    rownames        12   varchar
    2    ripley        <NA>  USArrests      Murder         8    double
    3    ripley        <NA>  USArrests     Assault         4   integer
    4    ripley        <NA>  USArrests    UrbanPop         4   integer
    5    ripley        <NA>  USArrests        Rape         8    double
    ... 12 more columns

    > sqlColumns(channel, "USArrests", special = TRUE)
      SCOPE COLUMN_NAME DATA_TYPE TYPE_NAME COLUMN_SIZE BUFFER_LENGTH
    1     2    rownames        12   varchar         255           255
      DECIMAL_DIGITS PSEUDO_COLUMN
    1             NA             1

    > sqlPrimaryKeys(channel, "USArrests")
      TABLE_CAT TABLE_SCHEM TABLE_NAME COLUMN_NAME KEY_SEQ PK_NAME
    1      <NA>        <NA>  USArrests    rownames       1 PRIMARY
    > sqlDrop(channel, "USArrests")
    > close(channel)

    ## End(Not run)

[Package RODBC version 1.3-18 Index]