The Ultimate Guide to High Availability with MariaDB

SELECT

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Syntax

SELECT
    [ALL | DISTINCT | DISTINCTROW]
    [HIGH_PRIORITY]
    [STRAIGHT_JOIN]
    [SQL_SMALL_RESULT] [SQL_BIG_RESULT] [SQL_BUFFER_RESULT]
    [SQL_CACHE | SQL_NO_CACHE] [SQL_CALC_FOUND_ROWS]
    select_expr [, select_expr ...]
    [ INTO OUTFILE 'file_name' [export_options]
    | INTO DUMPFILE 'file_name'
    | INTO var_name [, var_name] ]
    [ FROM table_references
      [WHERE where_condition]
      [GROUP BY {col_name | expr | position} [ASC | DESC], ... [WITH ROLLUP]]
      [HAVING where_condition]
      [ORDER BY {col_name | expr | position} [ASC | DESC], ...]
      [LIMIT {[offset,] row_count | row_count OFFSET offset}]
      [PROCEDURE procedure_name(argument_list)]
      [FOR UPDATE | LOCK IN SHARE MODE] ]

Description

SELECT is used to retrieve rows selected from one or more tables, and can include UNION statements and subqueries. See UNION, and http://dev.mysql.com/doc/refman/5.1/en/subqueries.html.

  • Each select_expr expression indicates a column or data that you want to retrieve. You must have at least one select expression. See Select Expressions below.
  • The FROM clause indicates the table or tables from which to retrieve rows. Use either a single table name or a JOIN expression. See JOIN for details.
  • The WHERE clause, if given, indicates the condition or conditions that rows must satisfy to be selected. where_condition is an expression that evaluates to true for each row to be selected. The statement selects all rows if there is no WHERE clause.
  • Use the ORDER BY clause to order the results.
  • Use the LIMIT clause allows you to restrict the results to only a certain number of rows, optionally with an offset.
  • Use the GROUP BY and HAVING clauses to group rows together when they have columns or computed values in common.

SELECT can also be used to retrieve rows computed without reference to any table.

Select Expressions

A SELECT statement must contain one or more select expressions, separated by commas. Each select expression can be one of the following:

  • The name of a column.
  • Any expression using functions and operators.
  • * to select all columns from all tables in the FROM clause.
  • tbl_name.* to select all columns from just the table tbl_name.

When specifying a column, you can either use just the column name or qualify the column name with the name of the table using tbl_name.col_name. The qualified form is useful if you are joining multiple tables in the FROM clause. If you do not qualify the column names when selecting from multiple tables, MariaDB will try to find the column in each table. It is an error if that column name exists in multiple tables.

You can quote column names using backticks. If you are qualifying column names with table names, quote each part separately as `tbl_name`.`col_name`.

If you use any grouping functions in any of the select expressions, all rows in your results will be implicitly grouped, as if you had used GROUP BY NULL.

ORDER BY

Use the ORDER BY clause to order the results that are returned from a SELECT statement. You can specify just a column or use any expression with functions. If you are using the GROUP BY clause, you can use grouping functions in ORDER BY. Ordering is done after grouping.

You can use multiple ordering expressions, separated by commas. Rows will be sorted by the first expression, then by the second expression if they have the same value for the first, and so on.

You can use the keywords ASC and DESC after each ordering expression to force that ordering to be ascending or descending, respectively. Ordering is ascending by default.

You can also use a single integer as the ordering expression. If you use an integer n, the results will be ordered by the nth column in the select expression.

When string values are compared, they are compared as if by the STRCMP function. STRCMP ignores trailing whitespace and may normalize characters and ignore case, depending on the collation in use.

LIMIT

Use the LIMIT clause to restrict the number of returned rows. When you use a single integer n with LIMIT, the first n rows will be returned. Use the ORDER BY clause to control which rows come first. You can also select a number of rows after an offset using either of the following:

LIMIT offset, row_count
LIMIT row_count OFFSET offset

When you provide an offset m with a limit n, the first m rows will be ignored, and the following n rows will be returned.

GROUP BY

Use the GROUP BY clause to group rows together that have the same value in one or more column, or the same computed value using expressions with any functions and operators except grouping functions. When you use a GROUP BY clause, you will get a single result row for each group of rows that have the same value for the expression given in GROUP BY.

When grouping rows, grouping values are compared as if by the = operator. For string values, the = operator ignores trailing whitespace and may normalize characters and ignore case, depending on the collation in use.

You can use any of the grouping functions in your select expression. Their values will be calculated based on all the rows that have been grouped together for each result row. If you select a non-grouped column or a value computed from a non-grouped column, it is undefined which row the returned value is taken from.

You can use multiple expressions in the GROUP BY clause, separated by commas. Rows are grouped together if they match on each of the expressions.

You can also use a single integer as the grouping expression. If you use an integer n, the results will be grouped by the nth column in the select expression.

The WHERE clause is applied before the GROUP BY clause. It filters non-aggregated rows before the rows are grouped together. To filter grouped rows based on aggregate values, use the HAVING clause. The HAVING clause takes any expression and evaluates it as a boolean, just like the WHERE clause. You can use grouping functions in the HAVING clause. As with the select expression, if you reference non-grouped columns in the HAVING clause, the behavior is undefined.

GROUP BY Examples

Consider the following table that records how many times each user has played and won a game:

CREATE TABLE plays (name VARCHAR(16), plays INT, wins INT);
INSERT INTO plays VALUES ("John", 20, 5), ("Robert", 22, 8), ("Wanda", 32, 8), ("Susan", 17, 3);

Get a list of win counts along with a count:

MariaDB> SELECT wins, COUNT(*) FROM plays GROUP BY wins;
+------+----------+
| wins | COUNT(*) |
+------+----------+
|    3 |        1 |
|    5 |        1 |
|    8 |        2 |
+------+----------+
3 rows in set (0.00 sec)

The GROUP BY expression can be a computed value, and can refer back to an identifer specified with AS. Get a list of win averages along with a count:

MariaDB> SELECT (wins / plays) AS winavg, COUNT(*) FROM plays GROUP BY winavg;
+--------+----------+
| winavg | COUNT(*) |
+--------+----------+
| 0.1765 |        1 |
| 0.2500 |        2 |
| 0.3636 |        1 |
+--------+----------+
3 rows in set (0.00 sec)

You can use any grouping function in the select expression. For each win average as above, get a list of the average play count taken to get that average:

MariaDB> SELECT (wins / plays) AS winavg, AVG(plays) FROM plays GROUP BY winavg;
+--------+------------+
| winavg | AVG(plays) |
+--------+------------+
| 0.1765 |    17.0000 |
| 0.2500 |    26.0000 |
| 0.3636 |    22.0000 |
+--------+------------+
3 rows in set (0.00 sec)

You can filter on aggregate information using the HAVING clause. The HAVING clause is applied after GROUP BY and allows you to filter on aggregate data that is not available to the WHERE clause. Restrict the above example to results that involve an average number of plays over 20:

MariaDB> SELECT (wins / plays) AS winavg, AVG(plays) FROM plays GROUP BY winavg HAVING AVG(plays) > 20;
+--------+------------+
| winavg | AVG(plays) |
+--------+------------+
| 0.2500 |    26.0000 |
| 0.3636 |    22.0000 |
+--------+------------+
2 rows in set (0.00 sec)

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