How to Dynamically Transpose a Table in SQL Using Prepared Statements?

How to Transpose a Table using Dynamic SQL

In SQL, transposing a table involves converting a row-oriented table to a column-oriented one. This can be useful for various scenarios, such as creating a user summary report.

One approach to achieve transposition in SQL is to use the CASE statement. Consider a table with columns: Id, UserId, FieldName, and FieldValue, as shown in the provided example.

SELECT t.userid,
       MAX(CASE WHEN t.fieldname = 'Username' THEN t.fieldvalue ELSE NULL END) AS Username,
       MAX(CASE WHEN t.fieldname = 'Password' THEN t.fieldvalue ELSE NULL END) AS Password,
       MAX(CASE WHEN t.fieldname = 'Email Address' THEN t.fieldvalue ELSE NULL END) AS Email
FROM t
GROUP BY t.userid

This query will return the transposed result, grouping users by UserId and assigning the corresponding field values to the Username, Password, and Email Address columns.

However, this approach requires defining CASE statements for each field, which can be tedious and inflexible. To make this dynamic, you can utilize MySQL's Prepared Statement (dynamic SQL) syntax.

SET @sql = CONCAT("SELECT userid");
-- Dynamically create the field selection part
SET @field_list = (
  SELECT GROUP_CONCAT(
    CONCAT('MAX(CASE WHEN t.fieldname = ''', fieldname, ''' THEN t.fieldvalue ELSE NULL END) AS ', fieldname)
  )
  FROM t
  GROUP BY userid
);
SET @sql = CONCAT(@sql, ", ", @field_list);

SET @sql = CONCAT(@sql, " FROM t GROUP BY userid");
PREPARE stmt FROM @sql;
EXECUTE stmt;

This query constructs a dynamic SQL statement using the result of a subquery to determine the field selection part. It uses the GROUP_CONCAT function to concatenate the CASE statements, ensuring that the dynamically constructed SQL query is valid.

By utilizing dynamic SQL, you can avoid manually defining CASE statements and retain flexibility, allowing your query to handle any number of user-defined fields.

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