Ruby DBI Read Operations
DBI provides various methods to fetch records from a database. Assuming dbh is a database handle and sth is a statement handle:
| No. | Method & Description |
|---|---|
| 1 | db.select_one( stmt, *bindvars ) => aRow | nil <br>Executes the stmt statement with bindvars bound to parameter markers. Returns the first row, or nil if the result set is empty. |
| 2 | db.select_all( stmt, *bindvars ) => [aRow, ...] | nil <br>db.select_all( stmt, *bindvars ) { |aRow| aBlock } <br>Executes the stmt statement with bindvars bound to parameter markers. Calling this method without a block returns an array containing all rows. If a block is given, it is called for each row. |
| 3 | sth.fetch => aRow | nil <br>Returns the next row. If there is no next row in the result, it returns nil. |
| 4 | sth.fetch { |aRow| aBlock } <br>Calls the given block for the remaining rows in the result set. |
| 5 | sth.fetch_all => [aRow, ...] <br>Returns all remaining rows in the result set as an array. |
| 6 | sth.fetch_many( count ) => [aRow, ...] <br>Returns the next count rows as an array. |
| 7 | sth.fetch_scroll( direction, offset=1 ) => aRow | nil <br>Returns the row specified by the direction and offset parameters. Other methods discard the offset parameter except for SQL_FETCH_ABSOLUTE and SQL_FETCH_RELATIVE. Possible values for the direction parameter are listed below. |
| 8 | sth.column_names => anArray <br>Returns the names of the columns. |
| 9 | column_info => [ aColumnInfo, ... ] <br>Returns an array of DBI::ColumnInfo objects. Each object stores information about a column, including its name, type, precision, and more. |
| 10 | sth.rows => rpc <br>Returns the number of rows affected by the statement, or nil if none. |
| 11 | sth.fetchable? => true | false <br>Returns true if rows can be fetched, otherwise false. |
| 12 | sth.cancel <br>Releases the resources held by the result set. After calling this method, you cannot fetch rows again unless you call execute again. |
| 13 | sth.finish <br>Releases the resources held by the prepared statement. After calling this method, you cannot call further operations on this object. |
Direction Parameter
The following values can be used for the direction parameter in the fetch_scroll method:
| Constant | Description |
|---|---|
| DBI::SQL_FETCH_FIRST | Fetches the first row. |
| DBI::SQL_FETCH_LAST | Fetches the last row. |
| DBI::SQL_FETCH_NEXT | Fetches the next row. |
| DBI::SQL_FETCH_PRIOR | Fetches the previous row. |
| DBI::SQL_FETCH_ABSOLUTE | Fetches the row at the specified offset. |
| DBI::SQL_FETCH_RELATIVE | Fetches the row at an offset from the current row. |
Example
The following example demonstrates how to fetch metadata for a statement. Assume we have an EMPLOYEE table.
#!/usr/bin/ruby -w
require "dbi"
begin
# Connect to the MySQL server
dbh = DBI.connect("DBI:Mysql:TESTDB:localhost",
"testuser", "test123")
sth = dbh.prepare("SELECT * FROM EMPLOYEE
WHERE INCOME > ?")
sth.execute(1000)
if sth.column_names.size == 0 then
puts "Statement has no result set"
printf "Number of rows affected: %d\n", sth.rows
else
puts "Statement has a result set"
rows = sth.fetch_all
printf "Number of rows: %d\n", rows.size
printf "Number of columns: %d\n", sth.column_names.size
sth.column_info.each_with_index do |info, i|
printf "--- Column %d (%s) ---\n", i, info["name"]
printf "sql_type: %s\n", info["sql_type"]
printf "type_name: %s\n", info["type_name"]
printf "precision: %s\n", info["precision"]
printf "scale: %s\n", info["scale"]
printf "nullable: %s\n", info["nullable"]
printf "indexed: %s\n", info["indexed"]
printf "primary: %s\n", info["primary"]
printf "unique: %s\n", info["unique"]
printf "mysql_type: %s\n", info["mysql_type"]
printf "mysql_type_name: %s\n", info["mysql_type_name"]
printf "mysql_length: %s\n", info["mysql_length"]
printf "mysql_max_length: %s\n", info["mysql_max_length"]
printf "mysql_flags: %s\n", info["mysql_flags"]
end
end
sth.finish
rescue DBI::DatabaseError => e
puts "An error occurred"
puts "Error code: #{e.err}"
puts "Error message: #{e.errstr}"
ensure
# Disconnect from the server
dbh.disconnect if dbh
end
This will produce the following result:
Statement has a result set
Number of rows: 5
Number of columns: 5
--- Column 0 (FIRST_NAME) ---
sql_type: 12
type_name: VARCHAR
precision: 20
scale: 0
nullable: true
indexed: false
primary: false
unique: false
mysql_type: 254
mysql_type_name: VARCHAR
mysql_length: 20
mysql_max_length: 4
mysql_flags: 0
--- Column 1 (LAST_NAME) ---
sql_type: 12
type_name: VARCHAR
precision: 20
scale: 0
nullable: true
indexed: false
primary: false
unique: false
mysql_type: 254
mysql_type_name: VARCHAR
mysql_length: 20
mysql_max_length: 5
mysql_flags: 0
--- Column 2 (AGE) ---
sql_type: 4
type_name: INTEGER
precision: 11
scale: 0
nullable: true
indexed: false
primary: false
unique: false
mysql_type: 3
mysql_type_name: INT
mysql_length: 11
mysql_max_length: 2
mysql_flags: 32768
--- Column 3 (SEX) ---
sql_type: 12
type_name: VARCHAR
precision: 1
scale: 0
nullable: true
indexed: false
primary: false
unique: false
mysql_type: 254
mysql_type_name: VARCHAR
mysql_length: 1
mysql_max_length: 1
mysql_flags: 0
--- Column 4 (INCOME) ---
sql_type: 6
type_name: FLOAT
precision: 12
scale: 31
nullable: true
indexed: false
primary: false
unique: false
mysql_type: 4
mysql_type_name: FLOAT
mysql_length: 12
mysql_max_length: 4
mysql_flags: 32768