fix: Change the way modifyColumn constructs its statements because th…

…e previous version was not including all necessary modifications

Every alter statement will be a separate item in the list returned by the modifyColumn function. The statement to alter the default value of a column will combine both the DROP CONSTRAINT and ADD CONSTRAINT statements in one String item, separated by a semicolon. The statement to alter the type and nullability will be part of one String item since that syntax is allowed in SQL Server, but it seems like it's not allowed for adding and dropping constraints.

exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/vendors/SQLServerDialect.kt

@@ -251,30 +251,52 @@ open class SQLServerDialect : VendorDialect(dialectName, SQLServerDataTypeProvid
         return columnDefault !in nonAcceptableDefaults
     }
 
-    override fun modifyColumn(column: Column<*>, columnDiff: ColumnDiff): List<String> =
-        super.modifyColumn(column, columnDiff).map { statement ->
-            if (columnDiff.defaults) {
-                val transaction = TransactionManager.current()
-                val tableName = transaction.identity(column.table)
-                val colName = transaction.identity(column)
-
-                val dropConstraint = "DROP CONSTRAINT IF EXISTS DF_${tableName}_$colName"
-
-                column.dbDefaultValue?.let {
-                    buildString {
-                        append(statement.substringBefore("MODIFY COLUMN") + dropConstraint)
+    override fun modifyColumn(column: Column<*>, columnDiff: ColumnDiff): List<String> {
+        val transaction = TransactionManager.current()
+        val tableName = transaction.identity(column.table)
+        val colName = transaction.identity(column)
+
+        val alterTablePart = "ALTER TABLE $tableName "
+
+        val statements = mutableListOf<String>()
+
+        statements.add(
+            buildString {
+                append(alterTablePart + "ALTER COLUMN $colName ${column.columnType.sqlType()}")
+
+                if (columnDiff.nullability) {
+                    val defaultValue = column.dbDefaultValue
+                    val isPKColumn = column.table.primaryKey?.columns?.contains(column) == true
+
+                    if (column.columnType.nullable ||
+                        (defaultValue != null && column.defaultValueFun == null && !currentDialect.isAllowedAsColumnDefault(defaultValue))) {
+                        append(" NULL")
+                    } else if (!isPKColumn) {
+                        append(" NOT NULL")
+                    }
+                }
+            })
+
+        if (columnDiff.defaults) {
+            val dropConstraint = "DROP CONSTRAINT IF EXISTS DF_${tableName}_$colName"
+
+            statements.add(
+                buildString {
+                    column.dbDefaultValue?.let {
+                        append(alterTablePart + dropConstraint)
                         append("; ")
                         append(
-                            statement.substringBefore("MODIFY COLUMN") +
+                            alterTablePart +
                                 "ADD CONSTRAINT DF_${tableName}_$colName DEFAULT ${SQLServerDataTypeProvider.processForDefaultValue(it)} for $colName"
                         )
-                    }
-                } ?: (statement.substringBefore("MODIFY COLUMN") + dropConstraint)
-            } else {
-                statement.replace("MODIFY COLUMN", "ALTER COLUMN")
-            }
+                    } ?: append(alterTablePart + dropConstraint)
+                }
+            )
         }
 
+        return statements
+    }
+
     override fun createDatabase(name: String): String = "CREATE DATABASE ${name.inProperCase()}"
 
     override fun dropDatabase(name: String) = "DROP DATABASE ${name.inProperCase()}"