Conditionally run an ALTER

gistfile1.sql

/* Emulate an IF EXISTS for ALTER TABLE ... DROP COLUMN */

SELECT COUNT(*) INTO @exists FROM `information_schema`.`COLUMNS` 
    WHERE `TABLE_SCHEMA` = DATABASE()
    AND `COLUMN_NAME` = 'link_type'
    AND `TABLE_NAME` = 'Social_Identity';

SET @query = IF(
    @exists = 1,
    'ALTER TABLE `Social_Identity` DROP COLUMN `link_type`',
    'SELECT "Column does not exist"');

PREPARE stmt FROM @query;

EXECUTE stmt;

/* Perform the ALTER */

ALTER TABLE `Social_Identity`
    ADD COLUMN `link_type` tinyint UNSIGNED NOT NULL DEFAULT 1
        COMMENT "Link type of connected user (default 1 = weak link)";