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https://dev.mysql.com/doc/refman/5.7/en/derived-tables.html
This does not work: SELECT AVG(SUM(column1)) FROM t1 GROUP BY column1; However, this query provides the desired information: SELECT AVG(sum_column1) FROM (SELECT SUM(column1) AS sum_column1 FROM t1 GROUP BY column1) AS t1; Notice that the column ...
https://dev.mysql.com/doc/refman/5.7/en/example-foreign-keys.html
A foreign key relationship involves a parent table that holds the initial column values, and a child table with column values that reference the parent column values. This following example relates parent and child tables through a single-column ...
https://dev.mysql.com/doc/refman/5.7/en/faqs-mysql-cluster.html
You must also remember to account for any column indexes as follows: Each primary key or hash index created for an NDBCLUSTER table requires 21−25 bytes per record. Spatial indexes are not supported (although spatial columns can be used). This ...
https://dev.mysql.com/doc/refman/5.7/en/floating-point-types.html
A precision from 0 to 23 results in a 4-byte single-precision FLOAT column. A precision from 24 to 53 results in an 8-byte double-precision DOUBLE column. For example, a column defined as FLOAT(7,4) looks like -999.9999 when displayed. MySQL ... The ...
https://dev.mysql.com/doc/refman/5.7/en/fulltext-search.html
Full-text indexes can be used only with InnoDB or MyISAM tables, and can be created only for CHAR, VARCHAR, or TEXT columns. MATCH() takes a comma-separated list that names the columns to be searched. This rules out, for example, a table column ...
https://dev.mysql.com/doc/refman/5.7/en/identifiers.html
Certain objects within MySQL, including database, table, index, column, alias, view, stored procedure, partition, tablespace, and other object names are known as identifiers. Section 9.2.1, “Identifier Length Limits”, indicates the maximum ...
https://dev.mysql.com/doc/refman/5.7/en/index-merge-optimization.html
In EXPLAIN output, the Index Merge method appears as index_merge in the type column. In this case, the key column contains a list of indexes used, and key_len contains a list of the longest key parts for those indexes. If all columns used in the ...
https://dev.mysql.com/doc/refman/5.7/en/information-schema-general-table-reference.html
The following table summarizes INFORMATION_SCHEMA general tables. For greater detail, see the individual table descriptions. Table 24.2 INFORMATION_SCHEMA General Tables Table Name Description CHARACTER_SETS Available character sets ...
https://dev.mysql.com/doc/refman/5.7/en/information-schema-innodb-buffer-pool-stats-table.html
The INNODB_BUFFER_POOL_STATS table has these columns: POOL_ID The buffer pool ID. Use the INFORMATION_SCHEMA COLUMNS table or the SHOW COLUMNS statement to view additional information about the columns of this table, including data types and default ... The INNODB_BUFFER_POOL_STATS table provides much of the same buffer pool information provided in SHOW ENGINE INNODB STATUS ...
https://dev.mysql.com/doc/refman/5.7/en/information-schema-innodb-cmp-table.html
The INNODB_CMP and INNODB_CMP_RESET tables have these columns: PAGE_SIZE The compressed page size in bytes. Use the INFORMATION_SCHEMA COLUMNS table or the SHOW COLUMNS statement to view additional information about the columns of this table, ...