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## Character sets

#### Character set refresher

MySQL defines a character set as "a set of symbols and their respective encodings". `ascii`, `latin1`, `utf8` and `utf16` are character sets supported by MySQL.

A collation is a set of rules for comparing characters in a character set. For example, a case-insensitive collation will make strings that only differ in case compare equal. All collations are associated to a single character set. For example, `utf8_spanish_ci` is a case-insensitive collation associated to the `utf8` character set. Every character set has a default collation, which will be used if a character set without a collation is specified. For example, `latin1_swedish_ci` is the default collation for the `latin1` character set.

You can find more information about these concepts in the official MySQL docs on character sets.

#### The connection character set and collation

Every connection has an associated character set and collation. The connection's character set determines the encoding for character strings sent to and retrieved from the server. This includes SQL query strings, string fields and column names in metadata. The connection's collation is used for string literal comparison.

Every session you establish can have its own different character set and collation. You can specify this in two ways:

- When calling `connection::connect` or `connection::handshake`, using `handshake_params::connection_collation`. You specify a numeric ID that identifies the collation to use, and your connection will use the character set associated to this collation. You can find collation IDs in the `<boost/mysql/mysql_collations.hpp>` and `<boost/mysql/mariadb_collations.hpp>` headers.

  The problem with this approach is that if you specify a collation ID that is unknown to the server (e.g. `utf8mb4_0900_ai_ci` for an old MySQL 5.7 server), the handshake operation will succeed but the connection will sillently fall back to the server's default character set, (usually `latin1`, which is not Unicode).

- At any time, issuing a `SET NAMES` SQL statement. For example, `"SET NAMES utf8mb4"` will set the current connection's character set to `utf8mb4` and the connection's collation to utf8mb4's default collation. If the character set is unknown, the `SET NAMES` statement will fail. You can use `connection::execute` to issue the statement:

    results result;
    conn.execute("SET NAMES utf8mb4", result);
    // Further operations can assume utf8mb4 as conn's charset

#### character_set_results and character_set_client

Both of the above methods are shortcuts to set several session-level variables. The ones that impact this library's behavior are:

- character_set_client determines the encoding that SQL statements sent to the server should have. This includes the SQL strings passed to `connection::execute` and `connection::prepare_statement`, and string parameters passed to `statement::bind`.

  Not all character sets are permissible in `character_set_client`. The server will accept setting this variable to any UTF-8 character set, but won't accept UTF-16.

- character_set_results determines the encoding that the server will use to send any kind of result, including string fields retrieved by `connection::execute`, metadata like `metadata::column_name` and error messages.

  Note that, when you define a string column with a character set (e.g. `"CREATE TABLE t1 (col1 VARCHAR(5) CHARACTER SET latin1)"`), the column's character set will be used for storage and comparisons, but not for client communication. If you set `character_set_results` to `utf16`, any field obtained by `SELECT`ing `col1` will be UTF16-encoded, and not latin1-encoded. Note also that `metadata::column_collation` reflects the charset and collation the server has converted the column to before sending it to the client. In the above example, `metadata::column_collation` will be the default collation for UTF16, rather than `latin1_swedish_ci`.

The table below summarizes the encoding used by each piece of functionality in this library:

Functionality | Encoding given by...
--- | ---
SQL query strings passed to `connection::execute` and `connection::prepare_statement` | `character_set_client`
String values passed as parameters to `statement::bind` | `character_set_client`
String fields retrieved by `connection::execute` or `connection::read_some_rows`: `field_view::as_string` `field_view::get_string` | `character_set_results`
Metadata strings: `metadata::database` `metadata::table` `metadata::original_table` `metadata::column_name` `metadata::original_column_name` | `character_set_results`
Server-generated error messages: `diagnostics::server_message` | `character_set_results`
Informational messages: `results::info` `execution_state::info` | ASCII. These can only contain ASCII characters and are always ASCII encoded. More info in this section.