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# Databases

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## Databases

If you need to access a relational database such as `sqlserver`, `mysql`, `postgres`, `oracle`, `cybase` etc. from your erlang application using the Erlang ODBC interface is a good way to go about it.

The Erlang ODBC application should work for any relational database that has an ODBC driver. But currently it is only regularly tested for `sqlserver` and `postgres`.

## Database independence

The Erlang ODBC interface is in principal database independent, e.i. an erlang program using the interface could be run without changes towards different databases. But as SQL is used it is alas possible to write database dependent programs. Even though SQL is an ANSI-standard meant to be database independent, different databases have proprietary extensions to SQL defining their own data types. If you keep to the ANSI data types you will minimize the problem. But unfortunately there is no guarantee that all databases actually treats the ANSI data types equivalently. For instance an installation of `Oracle Enterprise release 8.0.5.0.0 for unix` will accept that you create a table column with the ANSI data type `integer`, but when retrieving values from this column the driver reports that it is of type `SQL_DECIMAL(0, 38)` and not `SQL_INTEGER` as you may have expected.

Another obstacle is that some drivers do not support scrollable cursors which has the effect that the only way to traverse the result set is sequentially, with next, from the first row to the last, and once you pass a row you cannot go back. This means that some functions in the interface will not work together with certain drivers. A similar problem is that not all drivers support "row count" for select queries, hence resulting in that the function `select_count/[3,4]` will return `{ok, undefined}` instead of `{ok, NrRows}` where `NrRows` is the number of rows in the result set.

## Data types

The following is a list of the ANSI data types. For details turn to the ANSI standard documentation. Usage of other data types is of course possible, but you should be aware that this makes your application dependent on the database you are using at the moment.

- CHARACTER (size), CHAR (size)
- NUMERIC (precision, scale), DECIMAL (precision, scale), DEC (precision, scale ) precision - total number of digits, scale - total number of decimal places
- INTEGER, INT, SMALLINT
- FLOAT (precision)
- REAL
- DOUBLE PRECISION
- CHARACTER VARYING(size), CHAR VARYING(size)

When inputting data using sql_query/[2,3] the values will always be in string format as they are part of an SQL-query. Example:

          odbc:sql_query(Ref, "INSERT INTO TEST VALUES(1, 2, 3)").

#### Note

Note that when the value of the data to input is a string, it has to be quoted with `'`. Example:

    odbc:sql_query(Ref, "INSERT INTO EMPLOYEE VALUES(1, 'Jane', 'Doe', 'F')").

You may also input data using param_query/3,4 and then the input data will have the Erlang type corresponding to the ODBC type of the column.See ODBC to Erlang mapping

When selecting data from a table, all data types are returned from the database to the ODBC driver as an ODBC data type. The tables below shows the mapping between those data types and what is returned by the Erlang API.

ODBC Data Type | Erlang Data Type
--- | ---
SQL_CHAR(size) | String | Binary (configurable)
SQL_WCHAR(size) | Unicode binary encoded as UTF16 little endian.
SQL_NUMERIC(p,s) when (p >= 0 and p <= 9 and s == 0) | Integer
SQL_NUMERIC(p,s) when (p >= 10 and p <= 15 and s == 0) or (s <= 15 and s > 0) | Float
SQL_NUMERIC(p,s) when p >= 16 | String
SQL_DECIMAL(p,s) when (p >= 0 and p <= 9 and s == 0) | Integer
SQL_DECIMAL(p,s) when (p >= 10 and p <= 15 and s == 0) or (s <= 15 and s > 0) | Float
SQL_DECIMAL(p,s) when p >= 16 | String
SQL_INTEGER | Integer
SQL_SMALLINT | Integer
SQL_FLOAT | Float
SQL_REAL | Float
SQL_DOUBLE | Float
SQL_VARCHAR(size) | String | Binary (configurable)
SQL_WVARCHAR(size) | Unicode binary encoded as UTF16 little endian.

Table: Mapping of ODBC data types to the Erlang data types returned to the Erlang application.

ODBC Data Type | Erlang Data Type
--- | ---
SQL_TYPE_DATE | String
SQL_TYPE_TIME | String
SQL_TYPE_TIMESTAMP | {{YY, MM, DD}, {HH, MM, SS}}
SQL_LONGVARCHAR | String | Binary (configurable)
SQL_WLONGVARCHAR(size) | Unicode binary encoded as UTF16 little endian.
SQL_BINARY | String | Binary (configurable)
SQL_VARBINARY | String | Binary (configurable)
SQL_LONGVARBINARY | String | Binary (configurable)
SQL_TINYINT | Integer
SQL_BIT | Boolean

Table: Mapping of extended ODBC data types to the Erlang data types returned to the Erlang application.

#### Note

To find out which data types will be returned for the columns in a table use the function describe_table/2,3

## Batch handling

Grouping of SQL queries can be desirable in order to reduce network traffic. Another benefit can be that the data source sometimes can optimize execution of a batch of SQL queries.

Explicit batches an procedures described below will result in multiple results being returned from sql_query/[2,3]. while with parameterized queries only one result will be returned from param_query/[2,3].

### Explicit batches

The most basic form of a batch is created by semicolons separated SQL queries, for example:

    "SELECT * FROM FOO; SELECT * FROM BAR" or
    "INSERT INTO FOO VALUES(1,'bar'); SELECT * FROM FOO"

### Procedures

Different databases may also support creating of procedures that contains more than one SQL query. For example, the following SQLServer-specific statement creates a procedure that returns a result set containing information about employees that work at the department and a result set listing the customers of that department.

     CREATE PROCEDURE DepartmentInfo (@DepartmentID INT) AS
     SELECT * FROM Employee WHERE department = @DepartmentID
     SELECT * FROM Customers WHERE department = @DepartmentID

### Parameterized queries

To effectively perform a batch of similar queries, you can use parameterized queries. This means that you in your SQL query string will mark the places that usually would contain values with question marks and then provide lists of values for each parameter. For instance you can use this to insert multiple rows into the `EMPLOYEE` table while executing only a single SQL statement, for example code see "Using the Erlang API" section in the "Getting Started" chapter.

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