28.7.5 C API Data Structures

MYSQL

This structure represents handler for one database connection. It is used for almost all MySQL functions. Do not try to make a copy of a MYSQL structure. There is no guarantee that such a copy will be usable.

MYSQL_RES

This structure represents the result of a query that returns rows (SELECT, SHOW, DESCRIBE, EXPLAIN). The information returned from a query is called the result set in the remainder of this section.

MYSQL_ROW

This is a type-safe representation of one row of data. It is currently implemented as an array of counted byte strings. (You cannot treat these as null-terminated strings if field values may contain binary data, because such values may contain null bytes internally.) Rows are obtained by calling mysql_fetch_row().

MYSQL_FIELD

This structure contains metadata: information about a field, such as the field's name, type, and size. Its members are described in more detail later in this section. You may obtain the MYSQL_FIELD structures for each field by calling mysql_fetch_field() repeatedly. Field values are not part of this structure; they are contained in a MYSQL_ROW structure.

MYSQL_FIELD_OFFSET

This is a type-safe representation of an offset into a MySQL field list. (Used by mysql_field_seek().) Offsets are field numbers within a row, beginning at zero.

my_ulonglong

The type used for the number of rows and for mysql_affected_rows(), mysql_num_rows(), and mysql_insert_id(). This type provides a range of 0 to 1.84e19.

Some functions that return a row count using this type return -1 as an unsigned value to indicate an error or exceptional condition. You can check for -1 by comparing the return value to (my_ulonglong)-1 (or to (my_ulonglong)~0, which is equivalent).

On some systems, attempting to print a value of type my_ulonglong does not work. To print such a value, convert it to unsigned long and use a %lu print format. Example:

printf ("Number of rows: %lu\n",
        (unsigned long) mysql_num_rows(result));

my_bool

A boolean type, for values that are true (nonzero) or false (zero). The my_bool type was used before MySQL 8.0. As of MySQL 8.0, use the bool or int C type instead.

char * name

The name of the field, as a null-terminated string. If the field was given an alias with an AS clause, the value of name is the alias. For a procedure parameter, the parameter name.

char * org_name

The name of the field, as a null-terminated string. Aliases are ignored. For expressions, the value is an empty string. For a procedure parameter, the parameter name.

char * table

The name of the table containing this field, if it is not a calculated field. For calculated fields, the table value is an empty string. If the column is selected from a view, table names the view. If the table or view was given an alias with an AS clause, the value of table is the alias. For a UNION, the value is the empty string. For a procedure parameter, the procedure name.

char * org_table

The name of the table, as a null-terminated string. Aliases are ignored. If the column is selected from a view, org_table names the view. If the column is selected from a derived table, org_table names the base table. If a derived table wraps a view, org_table still names the base table. If the column is an expression, org_table is the empty string. For a UNION, the value is the empty string. For a procedure parameter, the value is the procedure name.

char * db

The name of the database that the field comes from, as a null-terminated string. If the field is a calculated field, db is an empty string. For a UNION, the value is the empty string. For a procedure parameter, the name of the database containing the procedure.

char * catalog

The catalog name. This value is always "def".

char * def

The default value of this field, as a null-terminated string. This is set only if you use mysql_list_fields().

unsigned long length

The width of the field. This corresponds to the display length, in bytes.

The server determines the length value before it generates the result set, so this is the minimum length required for a data type capable of holding the largest possible value from the result column, without knowing in advance the actual values that will be produced by the query for the result set.

unsigned long max_length

The maximum width of the field for the result set (the length in bytes of the longest field value for the rows actually in the result set). If you use mysql_store_result() or mysql_list_fields(), this contains the maximum length for the field. If you use mysql_use_result(), the value of this variable is zero.

The value of max_length is the length of the string representation of the values in the result set. For example, if you retrieve a FLOAT column and the “widest” value is -12.345, max_length is 7 (the length of '-12.345').

If you are using prepared statements, max_length is not set by default because for the binary protocol the lengths of the values depend on the types of the values in the result set. (See Section 28.7.9, “C API Prepared Statement Data Structures”.) If you want the max_length values anyway, enable the STMT_ATTR_UPDATE_MAX_LENGTH option with mysql_stmt_attr_set() and the lengths will be set when you call mysql_stmt_store_result(). (See Section 28.7.11.3, “mysql_stmt_attr_set()”, and Section 28.7.11.28, “mysql_stmt_store_result()”.)

unsigned int name_length

The length of name.

unsigned int org_name_length

The length of org_name.

unsigned int table_length

The length of table.

unsigned int org_table_length

The length of org_table.

unsigned int db_length

The length of db.

unsigned int catalog_length

The length of catalog.

unsigned int def_length

The length of def.

unsigned int flags

Bit-flags that describe the field. The flags value may have zero or more of the bits set that are shown in the following table.

Some of these flags indicate data type information and are superseded by or used in conjunction with the MYSQL_TYPE_xxx value in the field->type member described later:

NUM_FLAG indicates that a column is numeric. This includes columns with a type of MYSQL_TYPE_DECIMAL, MYSQL_TYPE_NEWDECIMAL, MYSQL_TYPE_TINY, MYSQL_TYPE_SHORT, MYSQL_TYPE_LONG, MYSQL_TYPE_FLOAT, MYSQL_TYPE_DOUBLE, MYSQL_TYPE_NULL, MYSQL_TYPE_LONGLONG, MYSQL_TYPE_INT24, and MYSQL_TYPE_YEAR.

NO_DEFAULT_VALUE_FLAG indicates that a column has no DEFAULT clause in its definition. This does not apply to NULL columns (because such columns have a default of NULL), or to AUTO_INCREMENT columns (which have an implied default value).

The following example illustrates a typical use of the flags value:

if (field->flags & NOT_NULL_FLAG)
    printf("Field cannot be null\n");

You may use the convenience macros shown in the following table to determine the boolean status of the flags value.

unsigned int decimals

The number of decimals for numeric fields, and the fractional seconds precision for temporal fields.

unsigned int charsetnr

An ID number that indicates the character set/collation pair for the field.

Normally, character values in result sets are converted to the character set indicated by the character_set_results system variable. In this case, charsetnr corresponds to the character set indicated by that variable. Character set conversion can be suppressed by setting character_set_results to NULL. In this case, charsetnr corresponds to the character set of the original table column or expression. See also Section 10.4, “Connection Character Sets and Collations”.

To distinguish between binary and nonbinary data for string data types, check whether the charsetnr value is 63. If so, the character set is binary, which indicates binary rather than nonbinary data. This enables you to distinguish BINARY from CHAR, VARBINARY from VARCHAR, and the BLOB types from the TEXT types.

charsetnr values are the same as those displayed in the Id column of the SHOW COLLATION statement or the ID column of the INFORMATION_SCHEMA COLLATIONS table. You can use those information sources to see which character set and collation specific charsetnr values indicate:

enum enum_field_types type

The type of the field. The type value may be one of the MYSQL_TYPE_ symbols shown in the following table.

The MYSQL_TYPE_TIME2, MYSQL_TYPE_DATETIME2, and MYSQL_TYPE_TIMESTAMP2) type codes are used only on the server side. Clients see the MYSQL_TYPE_TIME, MYSQL_TYPE_DATETIME, and MYSQL_TYPE_TIMESTAMP codes.

You can use the IS_NUM() macro to test whether a field has a numeric type. Pass the type value to IS_NUM() and it evaluates to TRUE if the field is numeric:

if (IS_NUM(field->type))
    printf("Field is numeric\n");

ENUM and SET values are returned as strings. For these, check that the type value is MYSQL_TYPE_STRING and that the ENUM_FLAG or SET_FLAG flag is set in the flags value.