Add Change Existing Column To Not Null as a MySQL TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1249 TILs and counting..._
+_1250 TILs and counting..._
 
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@@ -522,6 +522,7 @@ _1249 TILs and counting..._
 
 ### MySQL
 
+- [Change Existing Column To Not Null](mysql/change-existing-column-to-not-null.md)
 - [Default Username And Password For New Instance](mysql/default-username-and-password-for-new-instance.md)
 - [Display Output In A Vertical Format](mysql/display-output-in-a-vertical-format.md)
 - [Doing Date Math](mysql/doing-date-math.md)

mysql/change-existing-column-to-not-null.md

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+# Change Existing Column To Not Null
+
+Let's say you have an existing nullable column. You want to update the schema
+to enforce a `not null` constraint on that column. You can do that with an
+[`alter table`](https://dev.mysql.com/doc/refman/8.0/en/alter-table.html) DDL
+statement. You can do this with the `modify` or `change` option.
+
+With `modify` you redeclare the column definition with the options that you
+want. You'll need to know and specify the existing data type of that column.
+
+```sql
+alter table books modify publication_year int not null;
+```
+
+It is possible, but clumsy to do this with the `change` option because you
+declare the column name twice. That's because `change` is typically used to
+rename a column.
+
+```sql
+alter table books change publication_year publication_year int not null;
+```
+
+If you're updating a column for a table that already contains data, make a plan
+to backfill any existing records that have `null` for that column. Every record
+will need a value in that column before the `modify` can be applied.
+
+[source](https://stackoverflow.com/a/6305252/535590)