45 posts tagged with "Database"

The first thing to check in a MySQL database is which tables it has. SHOW TABLES answers that in one line, and INFORMATION_SCHEMA.TABLES adds the details: storage engine, collation, size and an approximate row count. This guide runs both against a real MySQL 9.7 database and explains the one number in there you shouldn't trust.

Unable to connect to any of the specified MySQL hosts is MySql.Data's message when it can't reach MySQL at all: it never got as far as checking a user name or password. The inner exception tells you more. I got it twice in a .NET C# program that connects to MySQL in Docker, for two different reasons, and four quick checks tell them apart.

Retrieval of the RSA public key is not enabled for insecure connections is one of the most confusing MySQL errors in .NET, because it comes and goes: the same connection string fails after MySQL restarts and works again later. The cause is MySQL's default sign-in method, caching_sha2_password, meeting a connection without TLS. This post shows it happening in a small .NET C# program, explains why it's intermittent, and fixes it the right way.

A migration that changed an enum column from numbers to names applied fine, but undoing it failed with "Incorrect integer value: 'Adjustment' for column 'Type' at row 3". EF Core generates a Down method that changes the column back to int, and MySQL can't turn the word Adjustment into a number. The fix is to make Down translate the names back to numbers before it changes the column.

I added a two-column index to a table, and EF Core's migration failed on MySQL with "Cannot drop index 'IX_stock_movements_ProductId': needed in a foreign key constraint". The new index starts with the foreign key column, so EF Core decided the old single-column index was no longer needed and dropped it first. MySQL needs an index behind every foreign key at every moment. Swapping two calls in the migration, in both directions, fixes it.

I changed a C# property from string? to a required string, and the EF Core migration for it failed on MySQL with "BLOB, TEXT, GEOMETRY or JSON column 'Description' can't have a default value". EF Core had added DEFAULT '' to fill the rows that were empty, and MySQL doesn't accept a plain default on a longtext column. The fix is a two-line edit to the migration: fill the empty rows yourself, then make the column NOT NULL without a default.

EF Core warned me that a migration "may result in the loss of data", and on MySQL that warning turned into an error: "Data too long for column 'Name' at row 2". Worse, the migration had already changed one column before it failed. This post shows how to find the values that don't fit, see exactly what the failed migration did, fix the data, and apply the migration again.

ERROR 1146 (42S02): Table '…' doesn't exist usually means exactly that. But sometimes the table is right there, just with different capital letters, and MySQL on Linux treats Products and products as two different names. This post shows the failing query, the setting that causes it, and how to choose table names that work the same on Linux, Windows and macOS.

ERROR 1264 (22003): Out of range value for column means the number you're saving doesn't fit the column's type. For a DECIMAL column, the limit isn't the number of decimals, which MySQL quietly rounds, but the number of digits before the decimal point. This post finds a column's exact limit, shows the difference between a value that's rounded and one that's refused, and fixes it from both sides: the column and the input.

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails means you inserted or updated a row that points at a parent which doesn't exist. I got it from my own mistake: I guessed that a new category had Id 3, because it was the third one I'd created. It wasn't. This post shows why the guess was wrong, how to find the right Id, how to insert without guessing, and how to answer 400 Bad Request from a .NET C# API.