Add Praefect command to show migration status

This adds the subcommand `praefect sql-migrate-status` to show
whether a migration has been applied.

1 files changed, 37 insertions, 0 deletions

diff --git a/doc/sql_migrations.md b/doc/sql_migrations.md
index 617da054a..a20a34f20 100644
--- a/doc/sql_migrations.md
+++ b/doc/sql_migrations.md
@@ -12,6 +12,43 @@ Praefect SQL migrations should be applied automatically when you deploy Praefect
 praefect -config /path/to/config.toml sql-migrate
 ```
 
+The migration will not start if there are any migrations in the database
+that are not known by the Praefect binary.
+
+Adding the `-ignore-unknown` will skip this check:
+
+```shell
+praefect -config /path/to/config.toml sql-migrate -ignore-unknown
+```
+
+## Showing the status of migrations
+
+To see which migrations have been applied, run:
+
+```
+praefect -config /path/to/config.toml sql-migrate-status
+```
+
+For example, the output may look like:
+
+```
++----------------------------------------+--------------------------------------+
+|               MIGRATION                |               APPLIED                |
++----------------------------------------+--------------------------------------+
+| 20200109161404_hello_world             | 2020-02-26 16:00:32.486129 -0800 PST |
+| 20200113151438_1_test_migration        | 2020-02-26 16:00:32.486871 -0800 PST |
+| 20200224220728_job_queue               | 2020-03-25 16:27:21.384917 -0700 PDT |
+| 20200324001604_add_sql_election_tables | no                                   |
+| 20200401010230_add_some_table          | unknown migration                    |
++----------------------------------------+--------------------------------------+
+```
+
+The first column contains the migration ID, and the second contains one of three items:
+
+1. The date on which the migration was applied
+2. `no` if the migration has not yet been applied
+3. `unknown migration` if the migration is not known by the current Praefect binary
+
 ## Rolling back migrations
 
 Rolling back SQL migrations in Praefect works a little differently