How to Dynamically Select Columns in MySQL Using Information Schema?

Dynamic Column Selection in MySQL

In MySQL, there might arise scenarios where you need to retrieve specific columns based on unknown table structure or anticipated schema changes. This poses a challenge when hardcoding column names in a SQL statement is impractical. This article explores a solution to this problem without resorting to selecting every column.

Utilizing Information Schema

The information_schema database contains metadata about all databases and tables on MySQL server. It includes a table called COLUMNS, which stores information about columns, including their names, data types, and table membership.

Constructing a Dynamic Query

To dynamically select column names, you can utilize a combination of MySQL functions and the information_schema.COLUMNS table. Here's a step-by-step approach:

1. Extract the column names of interest from information_schema.COLUMNS using a conditional statement (e.g., filtering for specific column prefixes).
2. Concatenate the selected column names into a string that can be used as part of a SELECT statement.
3. Create a prepared statement by placing the concatenated string into a variable and executing it with EXECUTE.

Example SQL Statement

The following SQL statement demonstrates how to dynamically select column names that start with "prefix" from the table "atable":

CREATE TABLE atable (
  prefix1 VARCHAR(10),
  prefix2 VARCHAR(10),
  notprefix3 INT,
  notprefix4 INT
);

INSERT INTO atable VALUES ('qwer qwer', 'qwerqwer', 1, 1);
INSERT INTO atable VALUES ('qwer qwer', 'asdfaasd', 1, 1);
INSERT INTO atable VALUES ('qwer qwer', 'qrt vbb', 1, 1);
INSERT INTO atable VALUES ('qwer qwer', 'sdfg sdg', 1, 1);

SELECT CONCAT('SELECT ', GROUP_CONCAT(c.COLUMN_NAME), ' FROM atable;')
INTO @query
FROM INFORMATION_SCHEMA.COLUMNS c
WHERE c.TABLE_NAME = 'atable'
  AND c.COLUMN_NAME LIKE 'prefix%'
ORDER BY c.ORDINAL_POSITION;

PREPARE stmt FROM @query;

EXECUTE stmt;

Output:

The output of this statement will be a single row containing the following SQL statement:

SELECT prefix1, prefix2 FROM atable;

By executing this statement, you will retrieve only the columns "prefix1" and "prefix2" from the "atable" table.

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