select * from a FULL OUTER JOIN b on a.a = b. If something in A doesn't have a corresponding datum in B, then the B portion is null, and vice versa. Select a.*, b.* from a,b where a.a(+) = b.b Ī full outer join will give you the union of A and B, i.e. select * from a RIGHT OUTER JOIN b on a.a = b.b Select a.*, b.* from a,b where a.a = b.b(+) Ī right outer join will give all rows in B, plus any common rows in A. This section discusses the functions and operators available for regular expression matching and illustrates, with examples, some of the special characters and constructs that can be used for regular expression. A regular expression is a powerful way of specifying a pattern for a complex search. RLIKE and NOT RLIKE are synonyms for REGEXP and NOT REGEXP, originally. Whether string matches regular expression. select * from a LEFT OUTER JOIN b on a.a = b.b The negative form NOT REGEXP also exists, as an alias for NOT (string REGEXP pattern). Select a.*, b.* from a,b where a.a = b.b Ī left outer join will give all rows in A, plus any common rows in B. Also note that using REGEXP prevents an index from being used and will result in a table scan or index scan. select * from a INNER JOIN b on a.a = b.b You could also rewrite it as the following to avoid also matching the empty string: REGEXP ' :digit:'. Note that (1,2) are unique to A, (3,4) are common, and (5,6) are unique to B.Īn inner join using either of the equivalent queries gives the intersection of the two tables, i.e. Suppose you have two tables, with a single column each, and data as follows: A B the inner part of a Venn diagram intersection.Īn outer join of A and B gives the results of A union B, i.e. The performance of like when using an index is very close to = (assuming the same number of rows returned).Īssuming you're joining on columns with no duplicates, which is a very common case:Īn inner join of A and B gives the result of A intersect B, i.e. This section discusses the functions and operators available for. SELECT * FROM t WHERE a LIKE '_agf' <<- cannot use an index A regular expression is a powerful way of specifying a pattern for a complex search. SELECT * FROM t WHERE a LIKE '%a%' <<- cannot use an index ![]() SELECT * FROM t WHERE a LIKE 'a%' <<- can use an index, depending on cardinality SELECT * FROM t WHERE a LIKE 'abc%' <<- can use an index These match specific character sets, as described in the following table. ![]() SELECT * FROM t WHERE a LIKE 'abc' <<- (case insensitive `=`) can use an index MySQLs regular expression capabilities also support POSIX character classes. ![]() LIKE can use an index if the wildcard is not the first char. No other indexes on the same table are better suited (MySQL can only use one index per table per subselect) Ĭonsidering these and some other more esoteric caveats an = comparison is much faster than a regexp.the values in the column have sufficient cardinality (if more than +/- 20% of the rows match, MySQL will not use an index, because in that case doing a full table scan is faster) In MySQL, the REGEXPLIKE() function is used to determine whether or not a string matches a regular expression.The regexp can never use an index in MySQL.
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