Ruby DBI Methods for Read Operation


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DBI provides several different methods to fetch records from the database. Assuming dbh is a database handle and sth is a statement handle −

Sr.No. Methods with Description
1

db.select_one( stmt, *bindvars ) = > aRow | nil

Executes the stmt statement with the bindvars binding beforehand to parameter markers. Returns the first row or nil if the result-set is empty.

2

db.select_all( stmt, *bindvars ) => [aRow, ...] | nil

db.select_all( stmt, *bindvars ){ |aRow| aBlock }

Executes the stmt statement with the bindvars binding beforehand to parameter markers. Calling this method without block returns an array containing all rows. If a block is given, this will be called for each row.

3

sth.fetch => aRow | nil

Returns the next row. Returns nil if no further rows are in the result-set.

4

sth.fetch { |aRow| aBlock }

Invokes the given block for the remaining rows of the result-set.

5

sth.fetch_all => [aRow, ...]

Returns all remaining rows of the result-set collected in an array.

6

sth.fetch_many( count ) => [aRow, ...]

Returns the next count rows collected in an [aRow, ...] array.

7

sth.fetch_scroll( direction, offset=1 ) => aRow | nil

Returns the row specified by the direction parameter and offset. Parameter offset is discarded for all but SQL_FETCH_ABSOLUTE and SQL_FETCH_RELATIVE. See a table below for possible values of direction parameter.

8

sth.column_names => anArray

Returns the names of the columns.

9

column_info => [ aColumnInfo, ... ]

Returns an array of DBI::ColumnInfo objects. Each object stores information about one column and contains its name, type, precision and more.

10

sth.rows => rpc

Returns the Row Processed Count of the executed statement or nil if no such exist.

11

sth.fetchable? => true | false

Returns true if it's possible to fetch rows, otherwise false.

12

sth.cancel

Frees the resources held by the result-set. After calling this method, it is no longer possible to fetch rows until you again call execute.

13

sth.finish

Frees the resources held by the prepared statement. After calling this method no further methods can be called onto this object.

The direction Parameter

Following values could be used for the direction Parameter of the fetch_scroll Method −

Sr.No. Constant & Description
1

DBI::SQL_FETCH_FIRST

Fetches first row.

2

DBI::SQL_FETCH_LAST

Fetches last row.

3

DBI::SQL_FETCH_NEXT

Fetches next row.

4

DBI::SQL_FETCH_PRIOR

Fetches previous row.

5

DBI::SQL_FETCH_ABSOLUTE

Fetches row at position offset.

6

DBI::SQL_FETCH_RELATIVE

Fetches the row that is offset rows away from the current.

Example

The following example shows how to get the metadata for a statement. Consider the EMPLOYEE table, which we created in the last chapter.

#!/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 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

ruby_database_access.htm

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