# DQL语法定义
#### DQL语句
- **语法定义**
  ```
  query:
    values
    | {
        select
        | selectWithoutFrom
        | query UNION [ ALL ] query
        | query EXCEPT query
        | query INTERSECT query
      }
      [ ORDER BY orderItem [, orderItem ]* ]
      [ LIMIT { count | ALL } ]
      [ OFFSET start { ROW | ROWS } ]
      [ FETCH { FIRST | NEXT } [ count ] { ROW | ROWS } ONLY]
  orderItem:
    expression [ ASC | DESC ]
  select:
    SELECT [ ALL | DISTINCT ]
    { * | projectItem [, projectItem ]* }
    FROM tableExpression
    [ WHERE booleanExpression ]
    [ GROUP BY { groupItem [, groupItem ]* } ]
    [ HAVING booleanExpression ]
    [ WINDOW windowName AS windowSpec [, windowName AS windowSpec ]* ]
  selectWithoutFrom:
    SELECT [ ALL | DISTINCT ]
    { * | projectItem [, projectItem ]* }
  projectItem:
    expression [ [ AS ] columnAlias ]
    | tableAlias . *
  tableExpression:
    tableReference [, tableReference ]*
    | tableExpression [ NATURAL ] [ LEFT | RIGHT | FULL ] JOIN tableExpression [ joinCondition ]
  joinCondition:
    ON booleanExpression
    | USING '(' column [, column ]* ')'
  tableReference:
    tablePrimary
    [ matchRecognize ]
    [ [ AS ] alias [ '(' columnAlias [, columnAlias ]* ')' ] ]
  tablePrimary:
    [ TABLE ] [ [ catalogName . ] schemaName . ] tableName
    | LATERAL TABLE '(' functionName '(' expression [, expression ]* ')' ')'
    | UNNEST '(' expression ')'
  values:
    VALUES expression [, expression ]*
  groupItem:
    expression
    | '(' ')'
    | '(' expression [, expression ]* ')'
    | CUBE '(' expression [, expression ]* ')'
    | ROLLUP '(' expression [, expression ]* ')'
    | GROUPING SETS '(' groupItem [, groupItem ]* ')'
  windowRef:
      windowName
    | windowSpec
  windowSpec:
      [ windowName ]
      '('
      [ ORDER BY orderItem [, orderItem ]* ]
      [ PARTITION BY expression [, expression ]* ]
      [
          RANGE numericOrIntervalExpression {PRECEDING}
        | ROWS numericExpression {PRECEDING}
      ]
      ')'
  matchRecognize:
        MATCH_RECOGNIZE '('
        [ PARTITION BY expression [, expression ]* ]
        [ ORDER BY orderItem [, orderItem ]* ]
        [ MEASURES measureColumn [, measureColumn ]* ]
        [ ONE ROW PER MATCH ]
        [ AFTER MATCH
              ( SKIP TO NEXT ROW
              | SKIP PAST LAST ROW
              | SKIP TO FIRST variable
              | SKIP TO LAST variable
              | SKIP TO variable )
        ]
        PATTERN '(' pattern ')'
        [ WITHIN intervalLiteral ]
        DEFINE variable AS condition [, variable AS condition ]*
        ')'
  measureColumn:
        expression AS alias
  pattern:
        patternTerm [ '|' patternTerm ]*
  patternTerm:
        patternFactor [ patternFactor ]*
  patternFactor:
        variable [ patternQuantifier ]
  patternQuantifier:
        '*'
    |   '*?'
    |   '+'
    |   '+?'
    |   '?'
    |   '??'
    |   '{' { [ minRepeat ], [ maxRepeat ] } '}' ['?']
    |   '{' repeat '}'
  ```
  
 
- **注意事项**
  1. SecMaster SQL兼容Flink 1.7.2版本SQL语法。
  
  
  
  2. Flink SQL 对于标识符（表、属性、函数名）有类似于 Java 的词法约定：
     - 不管是否引用标识符，都保留标识符的大小写。
     
     - 且标识符需区分大小写。
     
     - 与 Java 不一样的地方在于，通过反引号，可以允许标识符带有非字母的字符（如："SELECT a AS \`my field\` FROM t"）。
     
     
     字符串文本常量需要被单引号包起来（如 SELECT 'Hello World' ）。两个单引号表示转义（如 SELECT 'It''s me.'）。字符串文本常量支持 Unicode 字符，如需明确使用 Unicode 编码，请使用以下语法：
     - 使用反斜杠（\\）作为转义字符（默认）：SELECT U\&'\\263A'
     
     - 使用自定义的转义字符： SELECT U\&'#263A' UESCAPE '#'
      
  
  
  
  3. 慎用正则函数（REGEXP） 正则表达式是非常耗时的操作，对比加减乘除通常有百倍的性能开销，而且正则表达式在某些极端情况下可能会进入无限循环，导致作业阻塞，因此建议使用LIKE。正则函数包括：
     REGEXP
     REGEXP_REPLACE