The maximum row size for a table constrains the number (and possibly size) of columns because the total length of all columns cannot exceed this size. See Row Size Limits.

The storage requirements of individual columns constrain the number of columns that fit within a given maximum row size. Storage requirements for some data types depend on factors such as storage engine, storage format, and character set. See Section 11.8, “Data Type Storage Requirements”.

Storage engines may impose additional restrictions that limit table column count. For example, InnoDB has a limit of 1017 columns per table. See Section 15.6.1.6, “Limits on InnoDB Tables”. For information about other storage engines, see Chapter 16, Alternative Storage Engines.

Functional key parts (see Section 13.1.15, “CREATE INDEX Syntax”) are implemented as hidden virtual generated stored columns, so each functional key part in a table index counts against the table total column limit.

The internal representation of a MySQL table has a maximum row size limit of 65,535 bytes, even if the storage engine is capable of supporting larger rows. BLOB and TEXT columns only contribute 9 to 12 bytes toward the row size limit because their contents are stored separately from the rest of the row.

The maximum row size for an InnoDB table, which applies to data stored locally within a database page, is slightly less than half a page for 4KB, 8KB, 16KB, and 32KB innodb_page_size settings. For example, the maximum row size is slightly less than 8KB for the default 16KB InnoDB page size. For 64KB pages, the maximum row size is slightly less than 16KB. See Section 15.6.1.6, “Limits on InnoDB Tables”.

If a row containing variable-length columns exceeds the InnoDB maximum row size, InnoDB selects variable-length columns for external off-page storage until the row fits within the InnoDB row size limit. The amount of data stored locally for variable-length columns that are stored off-page differs by row format. For more information, see Section 15.10, “InnoDB Row Formats”.

Different storage formats use different amounts of page header and trailer data, which affects the amount of storage available for rows.

The MySQL maximum row size limit of 65,535 bytes is demonstrated in the following InnoDB and MyISAM examples. The limit is enforced regardless of storage engine, even though the storage engine may be capable of supporting larger rows.

mysql> CREATE TABLE t (a VARCHAR(10000), b VARCHAR(10000),
       c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),
       f VARCHAR(10000), g VARCHAR(6000)) ENGINE=InnoDB CHARACTER SET latin1;
ERROR 1118 (42000): Row size too large. The maximum row size for the used 
table type, not counting BLOBs, is 65535. This includes storage overhead, 
check the manual. You have to change some columns to TEXT or BLOBs

mysql> CREATE TABLE t (a VARCHAR(10000), b VARCHAR(10000),
       c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),
       f VARCHAR(10000), g VARCHAR(6000)) ENGINE=MyISAM CHARACTER SET latin1;
ERROR 1118 (42000): Row size too large. The maximum row size for the used 
table type, not counting BLOBs, is 65535. This includes storage overhead, 
check the manual. You have to change some columns to TEXT or BLOBs

In the following MyISAM example, changing a column to TEXT avoids the 65,535-byte row size limit and permits the operation to succeed because BLOB and TEXT columns only contribute 9 to 12 bytes toward the row size.

mysql> CREATE TABLE t (a VARCHAR(10000), b VARCHAR(10000),
       c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),
       f VARCHAR(10000), g TEXT(6000)) ENGINE=MyISAM CHARACTER SET latin1;
Query OK, 0 rows affected (0.02 sec)

The operation succeeds for an InnoDB table because changing a column to TEXT avoids the MySQL 65,535-byte row size limit, and InnoDB off-page storage of variable-length columns avoids the InnoDB row size limit.

mysql> CREATE TABLE t (a VARCHAR(10000), b VARCHAR(10000),
       c VARCHAR(10000), d VARCHAR(10000), e VARCHAR(10000),
       f VARCHAR(10000), g TEXT(6000)) ENGINE=InnoDB CHARACTER SET latin1;
Query OK, 0 rows affected (0.02 sec)

Storage for variable-length columns includes length bytes, which are counted toward the row size. For example, a VARCHAR(255) CHARACTER SET utf8mb3 column takes two bytes to store the length of the value, so each value can take up to 767 bytes.

The statement to create table t1 succeeds because the columns require 32,765 + 2 bytes and 32,766 + 2 bytes, which falls within the maximum row size of 65,535 bytes:

mysql> CREATE TABLE t1
       (c1 VARCHAR(32765) NOT NULL, c2 VARCHAR(32766) NOT NULL)
       ENGINE = InnoDB CHARACTER SET latin1;
Query OK, 0 rows affected (0.02 sec)

The statement to create table t2 fails because, although the column length is within the maximum length of 65,535 bytes, two additional bytes are required to record the length, which causes the row size to exceed 65,535 bytes:

mysql> CREATE TABLE t2
       (c1 VARCHAR(65535) NOT NULL)
       ENGINE = InnoDB CHARACTER SET latin1;
ERROR 1118 (42000): Row size too large. The maximum row size for the used 
table type, not counting BLOBs, is 65535. This includes storage overhead, 
check the manual. You have to change some columns to TEXT or BLOBs

Reducing the column length to 65,533 or less permits the statement to succeed.

mysql> CREATE TABLE t2
       (c1 VARCHAR(65533) NOT NULL)
       ENGINE = InnoDB CHARACTER SET latin1;
Query OK, 0 rows affected (0.01 sec)

For MyISAM tables, NULL columns require additional space in the row to record whether their values are NULL. Each NULL column takes one bit extra, rounded up to the nearest byte.

The statement to create table t3 fails because MyISAM requires space for NULL columns in addition to the space required for variable-length column length bytes, causing the row size to exceed 65,535 bytes:

mysql> CREATE TABLE t3
       (c1 VARCHAR(32765) NULL, c2 VARCHAR(32766) NULL)
       ENGINE = MyISAM CHARACTER SET latin1;
ERROR 1118 (42000): Row size too large. The maximum row size for the used 
table type, not counting BLOBs, is 65535. This includes storage overhead, 
check the manual. You have to change some columns to TEXT or BLOBs

For information about InnoDB NULL column storage, see Section 15.10, “InnoDB Row Formats”.

InnoDB restricts row size (for data stored locally within the database page) to slightly less than half a database page for 4KB, 8KB, 16KB, and 32KB innodb_page_size settings, and to slightly less than 16KB for 64KB pages.

The statement to create table t4 fails because the defined columns exceed the row size limit for a 16KB InnoDB page.

mysql> CREATE TABLE t4 (
       c1 CHAR(255),c2 CHAR(255),c3 CHAR(255),
       c4 CHAR(255),c5 CHAR(255),c6 CHAR(255),
       c7 CHAR(255),c8 CHAR(255),c9 CHAR(255),
       c10 CHAR(255),c11 CHAR(255),c12 CHAR(255),
       c13 CHAR(255),c14 CHAR(255),c15 CHAR(255),
       c16 CHAR(255),c17 CHAR(255),c18 CHAR(255),
       c19 CHAR(255),c20 CHAR(255),c21 CHAR(255),
       c22 CHAR(255),c23 CHAR(255),c24 CHAR(255),
       c25 CHAR(255),c26 CHAR(255),c27 CHAR(255),
       c28 CHAR(255),c29 CHAR(255),c30 CHAR(255),
       c31 CHAR(255),c32 CHAR(255),c33 CHAR(255)
       ) ENGINE=InnoDB ROW_FORMAT=DYNAMIC DEFAULT CHARSET latin1;
ERROR 1118 (42000): Row size too large (> 8126). Changing some columns to TEXT or BLOB may help.
In current row format, BLOB prefix of 0 bytes is stored inline.