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数据库系统有一个核心部件，那就是SQL解释器。用过mySQL的同学都知道，我们需要写一系列由SQL语言组成的代码来驱动数据库的运行，由此它就必须要有一个SQL语言解释器来解读SQL代码，然后根据代码的意图来驱动数据库执行相应的操作，本节我们就完成一个简单的SQL解释器。

解释器的原理基于编译原理，我在B站上专门有视频解释编译原理算法，因此我在这里不再赘述。实现一个解释器的首要步骤就是完成一个词法解析器，我在B站编译原理视频中实现过一个小型编译器(dragon-compiler)，因此我将其对应的词法解析器直接拿过来稍作改动，让其能对SQL代码进行词法解析。首先我们把其中的lexer部分直接拷贝到我们现在的项目，打开其中的token.go文件，我们首先修改其中token的定义，将SQL语言中关键字的定义添加进去，然后去除与 SQL无关的定义，修改后代码如下：

package lexer

type Tag uint32

const (
	//AND 对应SQL关键字
	AND Tag = iota + 256
	//BREAK
	//DO
	EQ
	FALSE
	GE
	ID
	//IF
	//ELSE
	INDEX
	LE
	INT
	FLOAT
	MINUS
	PLUS
	NE
	NUM
	//OR
	REAL
	//TRUE
	//WHILE
	LEFT_BRACE    // "{"
	RIGHT_BRACE   // "}"
	LEFT_BRACKET  //"("
	RIGHT_BRACKET //")"
	AND_OPERATOR
	OR_OPERATOR
	ASSIGN_OPERATOR
	NEGATE_OPERATOR
	LESS_OPERATOR
	GREATER_OPERATOR
	BASIC //对应int , float, bool, char 等类型定义
	//TEMP  //对应中间代码的临时寄存器变量
	//SEMICOLON

	//新增SQL对应关键字
	SELECT
	FROM
	WHERE
	INSERT
	INTO
	VALUES
	DELETE
	UPDATE
	SET
	CREATE
	TABLE
	INT
	VARCHAR
	VIEW
	AS
	INDEX
	ON
	COMMA
	STRING
	//SQL关键字定义结束
	EOF

	ERROR
)

var token_map = make(map[Tag]string)

func init() {
	//初始化SQL关键字对应字符串
	token_map[AND] = "AND"
	token_map[SELECT] = "SELECT"
	token_map[WHERE] = "where"
	token_map[INSERT] = "INSERT"
	token_map[INTO] = "INTO"
	token_map[VALUES] = "VALUES"
	token_map[DELETE] = "DELETE"
	token_map[UPDATE] = "UPDATE"
	token_map[SET] = "SET"
	token_map[CREATE] = "CREATE"
	token_map[TABLE] = "TABLE"
	token_map[INT] = "INT"
	token_map[VARCHAR] = "VARCHAR"
	token_map[VIEW] = "VIEW"
	token_map[AS] = "AS"
	token_map[INDEX] = "INDEX"
	token_MAP[ON] = "ON"
    token_map[COMMA] = ","
	token_map[BASIC] = "BASIC"
	//token_map[DO] = "do"
	//token_map[ELSE] = "else"
	token_map[EQ] = "EQ"
	token_map[FALSE] = "FALSE"
	token_map[GE] = "GE"
	token_map[ID] = "ID"
	//token_map[IF] = "if"
	token_map[INT] = "int"
	token_map[FLOAT] = "float"
	
	token_map[LE] = "<="
	token_map[MINUS] = "-"
	token_map[PLUS] = "+"
	token_map[NE] = "!="
	token_map[NUM] = "NUM"
	token_map[OR] = "OR"
	token_map[REAL] = "REAL"
	//token_map[TEMP] = "t"
	token_map[TRUE] = "TRUE"
	//token_map[WHILE] = "while"
	//token_map[DO] = "do"
	//token_map[BREAK] = "break"
	token_map[AND_OPERATOR] = "&"
	token_map[OR_OPERATOR] = "|"
	token_map[ASSIGN_OPERATOR] = "="
	token_map[NEGATE_OPERATOR] = "!"
	token_map[LESS_OPERATOR] = "<"
	token_map[GREATER_OPERATOR] = ">"
	token_map[LEFT_BRACE] = "{"
	token_map[RIGHT_BRACE] = "}"
	token_map[LEFT_BRACKET] = "("
	token_map[RIGHT_BRACKET] = ")"
	token_map[EOF] = "EOF"
	token_map[ERROR] = "ERROR"
	//token_map[SEMICOLON] = ";"

}

type Token struct {
	lexeme string
	Tag    Tag
}

func (t *Token) ToString() string {
	if t.lexeme == "" {
		return token_map[t.Tag]
	}

	return t.lexeme
}

func NewToken(tag Tag) Token {
	return Token{
		lexeme: "",
		Tag:    tag,
	}
}

func NewTokenWithString(tag Tag, lexeme string) *Token {
	return &Token{
		lexeme: lexeme,
		Tag:    tag,
	}
}

在上面代码修改中，我们把原来C语言的关键字去掉，增加了一系列SQL语言对应的关键字。打开文件word_token.go，做如下修改：

package lexer

type Word struct {
	lexeme string
	Tag    Token
}

func NewWordToken(s string, tag Tag) Word {
	return Word{
		lexeme: s,
		Tag:    NewToken(tag),
	}
}

func (w *Word) ToString() string {
	return w.lexeme
}

func GetKeyWords() []Word {
	key_words := []Word{}
	key_words = append(key_words, NewWordToken("||", OR))
	key_words = append(key_words, NewWordToken("==", EQ))
	key_words = append(key_words, NewWordToken("!=", NE))
	key_words = append(key_words, NewWordToken("<=", LE))
	key_words = append(key_words, NewWordToken(">=", GE))
	//增加SQL语言对应关键字
	key_words = append(key_words, NewWordToken("AND", AND))
	key_words = append(key_words, NewWordToken("SELECT", SELECT))
	key_words = append(key_words, NewWordToken("FROM", FROM))
	key_words = append(key_words, NewWordToken("INSERT", INSERT))
	key_words = append(key_words, NewWordToken("INTO", INTO))
	key_words = append(key_words, NewWordToken("VALUES", VALUES))
	key_words = append(key_words, NewWordToken("DELETE", DELETE))
	key_words = append(key_words, NewWordToken("UPDATE", UPDATE))
	key_words = append(key_words, NewWordToken("SET", SET))
	key_words = append(key_words, NewWordToken("CREATE", CREATE))
	key_words = append(key_words, NewWordToken("TABLE", TABLE))
	key_words = append(key_words, NewWordToken("INT", INT))
	key_words = append(key_words, NewWordToken("VARCHAR", VARCHAR))
	key_words = append(key_words, NewWordToken("VIEW", VIEW))
	key_words = append(key_words, NewWordToken("AS", AS))
	key_words = append(key_words, NewWordToken("INDEX", INDEX))
	key_words = append(key_words, NewWordToken("ON", ON))

	//key_words = append(key_words, NewWordToken("minus", MINUS))
	//key_words = append(key_words, NewWordToken("true", TRUE))
	//key_words = append(key_words, NewWordToken("false", FALSE))
	//key_words = append(key_words, NewWordToken("if", IF))
	//key_words = append(key_words, NewWordToken("else", ELSE))
	//增加while, do关键字
	//key_words = append(key_words, NewWordToken("while", WHILE))
	//key_words = append(key_words, NewWordToken("do", DO))
	//key_words = append(key_words, NewWordToken("break", BREAK))
	//添加类型定义
	//key_words = append(key_words, NewWordToken("int", BASIC))
	//key_words = append(key_words, NewWordToken("float", BASIC))
	//key_words = append(key_words, NewWordToken("bool", BASIC))
	//key_words = append(key_words, NewWordToken("char", BASIC))

	return key_words
}

这里的修改中也是把原来对应C语言的关键字去掉，增加上SQL语言的关键字定义。除了这些修改外，lexer的基本逻辑没有什么变化，其代码如下(lexer.go):

package lexer

import (
	"bufio"
	"strconv"
	"strings"
	"unicode"
)

type Lexer struct {
	Lexeme       string
	lexemeStack  []string
	tokenStack   []Token
	peek         byte
	Line         uint32
	reader       *bufio.Reader
	read_pointer int
	key_words    map[string]Token
}

func NewLexer(source string) Lexer {
	str := strings.NewReader(source)
	source_reader := bufio.NewReaderSize(str, len(source))
	lexer := Lexer{
		Line:      uint32(1),
		reader:    source_reader,
		key_words: make(map[string]Token),
	}

	lexer.reserve()

	return lexer
}

func (l *Lexer) ReverseScan() {
	/*
		back_len := len(l.Lexeme)
		只能un read 一个字节
		for i := 0; i < back_len; i++ {
			l.reader.UnreadByte()
		}
	*/
	if l.read_pointer > 0 {
		l.read_pointer = l.read_pointer - 1
	}

}

func (l *Lexer) reserve() {
	key_words := GetKeyWords()
	for _, key_word := range key_words {
		l.key_words[key_word.ToString()] = key_word.Tag
	}
}

func (l *Lexer) Readch() error {
	char, err := l.reader.ReadByte() //提前读取下一个字符
	l.peek = char
	return err
}

func (l *Lexer) ReadCharacter(c byte) (bool, error) {

	chars, err := l.reader.Peek(1)
	if err != nil {
		return false, err
	}

	peekChar := chars[0]
	if peekChar != c {
		return false, nil
	}

	l.Readch() //越过当前peek的字符
	return true, nil
}

func (l *Lexer) UnRead() error {
	return l.reader.UnreadByte()
}

func (l *Lexer) Scan() (Token, error) {

	if l.read_pointer < len(l.lexemeStack) {
		l.Lexeme = l.lexemeStack[l.read_pointer]
		token := l.tokenStack[l.read_pointer]
		l.read_pointer = l.read_pointer + 1
		return token, nil
	} else {
		l.read_pointer = l.read_pointer + 1
	}

	for {
		err := l.Readch()
		if err != nil {
			return NewToken(ERROR), err
		}

		if l.peek == ' ' || l.peek == '\t' {
			continue
		} else if l.peek == '\n' {
			l.Line = l.Line + 1
		} else {
			break
		}
	}

	l.Lexeme = ""

	switch l.peek {
	case ',':
		l.Lexeme = ","
		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token := NewToken(COMMA)
		l.tokenStack = append(l.tokenStack, token)
		return token, nil
	case '{':
		l.Lexeme = "{"
		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token := NewToken(LEFT_BRACE)
		l.tokenStack = append(l.tokenStack, token)
		return token, nil
	case '}':
		l.Lexeme = "}"
		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token := NewToken(RIGHT_BRACE)
		l.tokenStack = append(l.tokenStack, token)
		return token, nil
	case '+':
		l.Lexeme = "+"
		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token := NewToken(PLUS)
		l.tokenStack = append(l.tokenStack, token)
		return token, nil
	case '-':
		l.Lexeme = "-"
		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token := NewToken(MINUS)
		l.tokenStack = append(l.tokenStack, token)
		return token, nil
	case '(':
		l.Lexeme = "("
		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token := NewToken(LEFT_BRACKET)
		l.tokenStack = append(l.tokenStack, token)
		return token, nil
	case ')':
		l.Lexeme = ")"
		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token := NewToken(RIGHT_BRACKET)
		l.tokenStack = append(l.tokenStack, token)
		return token, nil
	case '&':
		l.Lexeme = "&"
		if ok, err := l.ReadCharacter('&'); ok {
			l.Lexeme = "&&"
			word := NewWordToken("&&", AND)
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			l.tokenStack = append(l.tokenStack, word.Tag)
			return word.Tag, err
		} else {
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			token := NewToken(AND_OPERATOR)
			l.tokenStack = append(l.tokenStack, token)
			return token, err
		}
	case '|':
		l.Lexeme = "|"
		if ok, err := l.ReadCharacter('|'); ok {
			l.Lexeme = "||"
			word := NewWordToken("||", OR)
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			l.tokenStack = append(l.tokenStack, word.Tag)
			return word.Tag, err
		} else {
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			token := NewToken(OR_OPERATOR)
			l.tokenStack = append(l.tokenStack, token)
			return token, err
		}

	case '=':
		l.Lexeme = "="
		if ok, err := l.ReadCharacter('='); ok {
			l.Lexeme = "=="
			word := NewWordToken("==", EQ)
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			l.tokenStack = append(l.tokenStack, word.Tag)
			return word.Tag, err
		} else {
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			token := NewToken(ASSIGN_OPERATOR)
			l.tokenStack = append(l.tokenStack, token)
			return token, err
		}

	case '!':
		l.Lexeme = "!"
		if ok, err := l.ReadCharacter('='); ok {
			l.Lexeme = "!="
			word := NewWordToken("!=", NE)
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			l.tokenStack = append(l.tokenStack, word.Tag)
			return word.Tag, err
		} else {
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			token := NewToken(NEGATE_OPERATOR)
			l.tokenStack = append(l.tokenStack, token)
			return token, err
		}

	case '<':
		l.Lexeme = "<"
		if ok, err := l.ReadCharacter('='); ok {
			l.Lexeme = "<="
			word := NewWordToken("<=", LE)
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			l.tokenStack = append(l.tokenStack, word.Tag)
			return word.Tag, err
		} else {
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			token := NewToken(LESS_OPERATOR)
			l.tokenStack = append(l.tokenStack, token)
			return token, err
		}

	case '>':
		l.Lexeme = ">"
		if ok, err := l.ReadCharacter('='); ok {
			l.Lexeme = ">="
			word := NewWordToken(">=", GE)
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			l.tokenStack = append(l.tokenStack, word.Tag)
			return word.Tag, err
		} else {
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			token := NewToken(GREATER_OPERATOR)
			l.tokenStack = append(l.tokenStack, token)
			return token, err
		}

    case '"':
		for {
			err := l.Readch()
			if l.peek == '"' {
				haveSeenQuote = false
				l.lexemeStack = append(l.lexemeStack, l.Lexeme)
				token := NewToken(STRING)
				l.tokenStack = append(l.tokenStack, token)
				return token, nil
			}

			if err != nil {
				panic("string no end with quota")
			}
			l.Lexeme += string(l.peek)
		}

	}

	if unicode.IsNumber(rune(l.peek)) {
		var v int
		var err error
		for {
			num, err := strconv.Atoi(string(l.peek))
			if err != nil {
				if l.peek != 0 { //l.peek == 0 意味着已经读完所有字符
					l.UnRead() //将字符放回以便下次扫描
				}

				break
			}
			v = 10*v + num
			l.Lexeme += string(l.peek)
			l.Readch()
		}

		if l.peek != '.' {
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			token := NewToken(NUM)
			token.lexeme = l.Lexeme
			l.tokenStack = append(l.tokenStack, token)
			return token, err
		}
		l.Lexeme += string(l.peek)
		l.Readch() //越过 "."

		x := float64(v)
		d := float64(10)
		for {
			l.Readch()
			num, err := strconv.Atoi(string(l.peek))
			if err != nil {
				if l.peek != 0 { //l.peek == 0 意味着已经读完所有字符
					l.UnRead() //将字符放回以便下次扫描
				}

				break
			}

			x = x + float64(num)/d
			d = d * 10
			l.Lexeme += string(l.peek)
		}
		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token := NewToken(REAL)
		token.lexeme = l.Lexeme
		l.tokenStack = append(l.tokenStack, token)
		return token, err
	}

	if unicode.IsLetter(rune(l.peek)) {
		var buffer []byte
		for {
			buffer = append(buffer, l.peek)
			l.Lexeme += string(l.peek)

			l.Readch()
			if !unicode.IsLetter(rune(l.peek)) {
				if l.peek != 0 { //l.peek == 0 意味着已经读完所有字符
					l.UnRead() //将字符放回以便下次扫描
				}
				break
			}
		}

		s := string(buffer)
		token, ok := l.key_words[s]
		if ok {
			l.lexemeStack = append(l.lexemeStack, l.Lexeme)
			l.tokenStack = append(l.tokenStack, token)
			return token, nil
		}

		l.lexemeStack = append(l.lexemeStack, l.Lexeme)
		token = NewToken(ID)
		token.lexeme = l.Lexeme
		l.tokenStack = append(l.tokenStack, token)
		return token, nil
	}

	return NewToken(EOF), nil
}

为了节省篇幅，这里我没有把所有文件对应改动都贴出来，请在B站搜索”coding迪斯尼”查看详细内容，下面我们调用上面实现的代码试试效果，在main.go中添加如下测试代码：

import (
	"fmt"
	"lexer"
)

func main() {
	sqlLexer := lexer.NewLexer("select name , sex from student where age > 20")
	var tokens []*lexer.Token
	tokens = append(tokens, lexer.NewTokenWithString(lexer.SELECT, "select"))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.ID, "name"))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.COMMA, ","))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.ID, "sex"))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.FROM, "from"))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.ID, "student"))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.WHERE, "where"))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.ID, "age"))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.GREATER_OPERATOR, ">"))
	tokens = append(tokens, lexer.NewTokenWithString(lexer.NUM, "20"))

	for _, tok := range tokens {
		sqlTok, err := sqlLexer.Scan()
		if err != nil {
			fmt.Println("lexer error")
			break
		}

		if sqlTok.Tag != tok.Tag {
			errText := fmt.Sprintf("token err, expect: %v, but got %v\n", tok, sqlTok)
			fmt.Println(errText)
			break
		}
	}

	fmt.Println("lexer testing pass...")
}

通过运行可以发现，最后一句”lexer testing pass…”能正常打印出来，因此词法解析器的基本逻辑是正确的。接下来看看语法解析的实现，基于篇幅所限，这里我们只处理SQL的一小部分，有兴趣的同学可以自行补全我们这里完成的SQL解释器，首先我们先定义要解析的SQL语法部分：

FIELD -> ID

CONSTANT -> STRING | NUM

EXPRESSION -> FIELD | CONSTANT

TERM -> EXPRESSION EQ EXPRESSION

PREDICATE -> TERM (AND PREDICATE)?

QUERY -> SELECT SELECT_LIST FROM TABLE_LIST (WHERE PREDICATE)?

SELECTION_LIST -> FIELD (COMMA SELECTION_LIST)?

TABLE_LIST -> ID (COMMA TABLE_LIST)?

UPDATE_COMMAND -> INSERT_COMMAND | DELETE_COMMAND | MODIFY_COMMAND | CREATE_COMMAND

CREATE_COMMAND -> CREATE_TABLE | CREATE_VIEW | CREATE_INDEX

INSERT_COMMAND -> INSERT INTO ID LEFT_BRACE FIELD_LIST RIGHT_BRACE VALUES CONSTANT_LIST

FIELD_LIST -> FIELD (COMMA FIELD_LIST)?

CONSTANT_LIST -> CONSTANT (COMMA CONSTANT_LIST)?

DELETE_COMMAND -> DELETE FROM ID (WHERE PREDICATE)?

MODIFY_COMMAND -> UPDATE ID SET FIELD EQ EXPRESSION (WHERE PREDICATE)?

CREATE_TABLE -> CREATE TABLE FIELD_DEFS

FIELD_DEFS -> FIELD_DEF (COMMA FIELD_DEFS)?

FIELD_DEF -> ID TYPE_DEF

TYPE_DEF -> INT | VARCHAR LEFT_BRACE NUM RIGHT_BRACE

CREATE_VIEW -> CREATE VIEW ID AS QUERY

CREATE_INDEX -> CREATE INDEX ID ON ID LEFT_BRACE FIELD RIGHT_BRACE

接下来我们看看如何通过上面语法规则对SQL代码进行解析。这里我们采用自顶向下的递归式解析法，具体算法过程可以参考我在b站的编译原理视频。在工程中新建一个文件夹叫parser，然后再里面添加parser.go文件，为了简单起见，我们一次完成一小部分，然后调用完成的代码看看结果是否正确，首先我们完成TERM这条规则的解析，代码如下：

package parser

import (
	"lexer"
	"query"
	"strconv"
	"strings"
)

type SQLParser struct {
	sqlLexer lexer.Lexer
}

func NewSQLParser(s string) *SQLParser {
	return &SQLParser{
		sqlLexer: lexer.NewLexer(s),
	}
}

func (p *SQLParser) Field() (lexer.Token, string) {
	tok, err := p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}
	if tok.Tag != lexer.ID {
		panic("Tag of FIELD is no ID")
	}

	return tok, p.sqlLexer.Lexeme
}

func (p *SQLParser) Constant() *query.Constant {
	tok, err := p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}

	switch tok.Tag {
	case lexer.STRING:
		s := strings.Clone(p.sqlLexer.Lexeme)
		return query.NewConstantWithString(&s)
		break
	case lexer.NUM:
		//注意堆栈变量在函数执行后是否会变得无效
		v, err := strconv.Atoi(p.sqlLexer.Lexeme)
		if err != nil {
			panic("string is not a number")
		}
		return query.NewConstantWithInt(&v)
		break
	default:
		panic("token is not string or num when parsing constant")
	}

	return nil
}

func (p *SQLParser) Expression() *query.Expression {
	tok, err := p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}

	if tok.Tag == lexer.ID {
		p.sqlLexer.ReverseScan()
		_, str := p.Field()
		return query.NewExpressionWithString(str)
	} else {
		p.sqlLexer.ReverseScan()
		constant := p.Constant()
		return query.NewExpressionWithConstant(constant)
	}
}

func (p *SQLParser) Term() *query.Term {
	lhs := p.Expression()
	tok, err := p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}
	if tok.Tag != lexer.ASSIGN_OPERATOR {
		panic("should have = in middle of term")
	}

	rhs := p.Expression()
	return query.NewTerm(lhs, rhs)
}

TERM规则解析的是类似这样的表达式”age < 20″, “name = ‘jim’ “等尝出现在where 右边的表达式。我们调用上面解析代码进行测试看看，在main.go中输入如下代码：

import (
	"fmt"
	"parser"
)

func main() {
	sqlParser := parser.NewSQLParser("age = 20")
	term := sqlParser.Term()
	s := fmt.Sprintf("term: %v\n", term)
	fmt.Println(s)
}

请到B站查看我对上面代码进行调试演示的过程，这样更容易理解和吃透代码逻辑。接下来我们继续完成如下语法的解析：

PREDICATE -> TERM (AND PREDICATE)?

QUERY -> SELECT SELECT_LIST FROM TABLE_LIST (WHERE PREDICATE)?

SELECTION_LIST -> FIELD (COMMA SELECTION_LIST)?

TABLE_LIST -> ID (COMMA TABLE_LIST)?

这里需要注意的是PREDICATE对应的是where 后面的部分，例如where a > b and c < d，这条语句中”a>b and c < d”就是语法中的PREDICATE，对应代码如下：

func (p *SQLParser) Predicate() *query.Predicate {
	//predicate 对应where 语句后面的判断部分，例如where a > b and c < b
	//这里的a > b and c < b就是predicate
	pred := query.NewPredicateWithTerms(p.Term())
	tok, err := p.sqlLexer.Scan()
	// 如果语句已经读取完则直接返回
	if err != nil && fmt.Sprint(err) != "EOF" {
		panic(err)
	}

	if tok.Tag == lexer.AND {
		pred.ConjoinWith(p.Predicate())
	} else {
		p.sqlLexer.ReverseScan()
	}

	return pred
}

func (p *SQLParser) Query() *QueryData {
	//query 解析select 语句
	tok, err := p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}

	if tok.Tag != lexer.SELECT {
		panic("token is not select")
	}

	fields := p.SelectList()
	tok, err = p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}

	if tok.Tag != lexer.FROM {
		panic("token is not from")
	}

	//获取select语句作用的表名
	tables := p.TableList()
	//判断select语句是否有where子句
	tok, err = p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}

	pred := query.NewPredicate()
	if tok.Tag == lexer.WHERE {
		pred = p.Predicate()
	} else {
		p.sqlLexer.ReverseScan()
	}

	return NewQueryData(fields, tables, pred)
}

func (p *SQLParser) SelectList() []string {
	//SELECT_LIST 对应select关键字后面的列名称
	l := make([]string, 0)
	_, field := p.Field()
	l = append(l, field)

	tok, err := p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}
	if tok.Tag == lexer.COMMA {
		//selct 多个列，每个列由逗号隔开
		selectList := p.SelectList()
		l = append(l, selectList...)
	} else {
		p.sqlLexer.ReverseScan()
	}

	return l
}

func (p *SQLParser) TableList() []string {
	//TBALE_LSIT对应from后面的表名
	l := make([]string, 0)
	tok, err := p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}
	if tok.Tag != lexer.ID {
		panic("token is not id")
	}

	l = append(l, p.sqlLexer.Lexeme)
	tok, err = p.sqlLexer.Scan()
	if err != nil {
		panic(err)
	}
	if tok.Tag == lexer.COMMA {
		tableList := p.TableList()
		l = append(l, tableList...)
	} else {
		p.sqlLexer.ReverseScan()
	}

	return l
}

新增一个go文件名为query_data.go，我们使用数据结构QueryData来存储select语句的解析结果，起内容如下：

package parser

//QueryData 用来描述select语句的操作信息
import (
	"query"
)

type QueryData struct {
	fields []string
	tables []string
	pred   *query.Predicate
}

func NewQueryData(fields []string, tables []string, pred *query.Predicate) *QueryData {
	return &QueryData{
		fields: fields,
		tables: tables,
		pred:   pred,
	}
}

func (q *QueryData) Fields() []string {
	return q.fields
}

func (q *QueryData) Tables() []string {
	return q.tables
}

func (q *QueryData) Pred() *query.Predicate {
	return q.pred
}

func (q *QueryData) ToString() string {
	result := "select "
	for _, fldName := range q.fields {
		result += fldName + ", "
	}

	// 去掉最后一个逗号
	result = result[:len(result)-1]
	result += " from "
	for _, tableName := range q.tables {
		result += tableName + ", "
	}
	// 去掉最后一个逗号
	result = result[:len(result)-1]
	predStr := q.pred.ToString()
	if predStr != "" {
		result += " where " + predStr
	}

	return result
}

假设有SQL语句如下：

select age, name, sex from student, department where age = 20 and sex = "male" 

那么我们就能调用上面代码中的Query来启动解析，其中select后面的列表名也就是”age, name, sex”由函数SelectList负责解析，from 后面的表名由函数TableList 负责解析，where后面的内容由Predicate解析，其中他会把age = 20 和 sex = “male“ 解析成expression，我们在main.go中添加如下代码，以便调用起上面代码：

import (
	"fmt"
	"parser"
)

func main() {
	sqlParser := parser.NewSQLParser("select age, name, sex from student, department where age = 20 and sex = \"male\" ")
	queryData := sqlParser.Query()
	fmt.Println(queryData.ToString())
}

具体的调试演示过程请大家参看b站上的视频，通过调试演示我们才能更好的理解解析逻辑。由于本节内容较多，我们将其分割成几个小节来处理。