sql server left out join

在 SQL Server 中，你可以使用 LEFT OUTER JOIN（左外连接）来将两个或多个表连接起来，并返回左表中的所有行以及与右表匹配的行。左外连接保留左表的所有行，即使在右表中没有匹配的行也是如此。

下面是一个使用 LEFT OUTER JOIN 的示例查询：

SELECT *
FROM Table1
LEFT OUTER JOIN Table2 ON Table1.column = Table2.column;

在这个示例中，Table1 是左表，Table2 是右表，column 是它们之间的共同列。这个查询将返回 Table1 中的所有行，以及与 Table1.column 匹配的 Table2 中的行。如果在 Table2 中没有匹配的行，则相应的列将包含 NULL 值。

请根据你的实际表名和列名修改上述查询以适应你的需求。

相关问题

Java ，druid 解析sql，并通过 feign接口查询到的表元数据，替换包含left join 的select sql语句中的*，支持mysql、hive、postgresql、oracle、sqlserver等多种数据库。需要写出代码

下面是使用Java和Druid实现解析SQL语句，并通过Feign接口查询表元数据，替换包含left join的select语句中的*的示例代码：

import com.alibaba.druid.sql.SQLUtils;
import com.alibaba.druid.sql.ast.SQLExpr;
import com.alibaba.druid.sql.ast.SQLJoinTableSource;
import com.alibaba.druid.sql.ast.SQLStatement;
import com.alibaba.druid.sql.ast.expr.SQLIdentifierExpr;
import com.alibaba.druid.sql.ast.statement.SQLSelect;
import com.alibaba.druid.sql.ast.statement.SQLSelectItem;
import com.alibaba.druid.sql.ast.statement.SQLSelectQueryBlock;
import com.alibaba.druid.sql.ast.statement.SQLTableSource;
import com.alibaba.druid.sql.dialect.mysql.ast.statement.MySqlSelectQueryBlock;
import com.alibaba.druid.sql.dialect.mysql.visitor.MySqlOutputVisitor;
import com.alibaba.druid.util.JdbcConstants;

import java.util.List;
import java.util.regex.Matcher;
import java.util.regex.Pattern;

public class SqlParser {
    private static final String DB_TYPE_MYSQL = "mysql";
    private static final String DB_TYPE_ORACLE = "oracle";
    private static final String DB_TYPE_POSTGRESQL = "postgresql";
    private static final String DB_TYPE_SQLSERVER = "sqlserver";
    private static final String DB_TYPE_HIVE = "hive";

    private static final Pattern LEFT_JOIN_PATTERN = Pattern.compile("(?i)\\b(left join)\\b");

    private FeignClient feignClient; // Feign接口

    public void parseAndReplaceSql(String sql, String dbType) {
        // 解析SQL语句
        List<SQLStatement> sqlStatements = SQLUtils.parseStatements(sql, dbType);

        for (SQLStatement statement : sqlStatements) {
            // 只处理SELECT语句
            if (statement instanceof SQLSelect) {
                SQLSelect select = (SQLSelect) statement;
                SQLSelectQueryBlock queryBlock = (SQLSelectQueryBlock) select.getQuery();

                // 获取所有SELECT项
                List<SQLSelectItem> selectItems = queryBlock.getSelectList();

                // 判断是否包含LEFT JOIN
                boolean hasLeftJoin = false;
                SQLTableSource from = queryBlock.getFrom();
                if (from instanceof SQLJoinTableSource) {
                    SQLJoinTableSource join = (SQLJoinTableSource) from;
                    if (join.getJoinType().getName().equalsIgnoreCase("left join")) {
                        hasLeftJoin = true;
                    }
                }

                // 如果包含LEFT JOIN，则替换SELECT项中的*
                if (hasLeftJoin) {
                    for (int i = 0; i < selectItems.size(); i++) {
                        SQLSelectItem item = selectItems.get(i);
                        SQLExpr expr = item.getExpr();
                        if (expr instanceof SQLIdentifierExpr && ((SQLIdentifierExpr) expr).getName().equals("*")) {
                            // 调用Feign接口查询表元数据，替换SELECT项中的*
                            String tableName = ((SQLIdentifierExpr) from).getName(); // 获取表名
                            List<TableColumn> columns = feignClient.getTableColumns(tableName);
                            StringBuilder sb = new StringBuilder();
                            for (TableColumn column : columns) {
                                sb.append(column.getColumnName()).append(",");
                            }
                            sb.deleteCharAt(sb.length() - 1);
                            item.setExpr(SQLUtils.toSQLExpr(sb.toString(), dbType));
                            selectItems.set(i, item);
                        }
                    }
                }
            }
        }

        // 生成替换后的SQL语句
        String resultSql = SQLUtils.toSQLString(sqlStatements, dbType, new MySqlOutputVisitor());
        System.out.println(resultSql);
    }
}

// 表元数据
class TableColumn {
    private String columnName;
    private String columnType;

    // getter/setter
}

// Feign接口
@FeignClient(name = "metadata-service")
public interface FeignClient {
    @GetMapping("/tableColumns")
    List<TableColumn> getTableColumns(@RequestParam("tableName") String tableName);
}

需要注意的是，上述代码中的`FeignClient`是使用Spring Cloud Feign实现的，需要在项目中引入相应的依赖。同时，`getTableColumns`方法需要根据实际情况进行修改，具体实现可以参考Feign的使用文档。

Java ，druid 解析sql，并通过 feign接口查询到的表元数据，替换包含left join的insert into table ass_data select sql，支持mysql、hive、postgresql、oracle、sqlserver等多种数据库。需要写出代码

以下是一个示例代码，解析 SQL 并替换其中的 left join：

import java.sql.*;
import java.util.*;
import java.util.regex.*;

import com.alibaba.druid.pool.*;
import com.alibaba.druid.sql.*;
import com.alibaba.druid.sql.ast.*;
import com.alibaba.druid.sql.ast.expr.*;
import com.alibaba.druid.sql.ast.statement.*;
import com.alibaba.druid.sql.dialect.mysql.visitor.*;
import com.alibaba.druid.sql.dialect.oracle.visitor.*;
import com.alibaba.druid.sql.dialect.postgresql.visitor.*;
import com.alibaba.druid.sql.dialect.sqlserver.visitor.*;
import com.alibaba.druid.sql.visitor.*;

import feign.*;
import feign.jackson.*;

public class SQLParser {
    private static final Pattern LEFT_JOIN_PATTERN = Pattern.compile("\\bleft join\\b", Pattern.CASE_INSENSITIVE);

    public static void main(String[] args) throws Exception {
        String sql = "insert into table ass_data select a.*, b.* from table_a a left join table_b b on a.id = b.a_id";
        String replacedSql = replaceLeftJoin(sql);
        System.out.println(replacedSql);

        // 使用 Feign 查询表元数据
        String url = "http://localhost:8080/metaData";
        String databaseType = "mysql"; // 数据库类型
        String tableName = "table_a"; // 表名
        Feign.Builder builder = Feign.builder().encoder(new JacksonEncoder()).decoder(new JacksonDecoder());
        MetaDataService metaDataService = builder.target(MetaDataService.class, url);
        TableMetaData tableMetaData = metaDataService.getTableMetaData(databaseType, tableName);
        System.out.println(tableMetaData);
    }

    public static String replaceLeftJoin(String sql) throws Exception {
        SQLStatementParser parser = new SQLStatementParser(sql);
        SQLStatement statement = parser.parseStatement();
        if (statement instanceof SQLInsertStatement) {
            SQLInsertStatement insertStatement = (SQLInsertStatement) statement;
            SQLSelectQuery selectQuery = insertStatement.getQuery().getQuery();
            if (selectQuery instanceof SQLSelectQueryBlock) {
                SQLSelectQueryBlock queryBlock = (SQLSelectQueryBlock) selectQuery;
                List<SQLJoinTableSource> joinList = queryBlock.getJoinList();
                for (SQLJoinTableSource join : joinList) {
                    if (join.getJoinType() == SQLJoinTableSource.JoinType.LEFT_OUTER_JOIN) {
                        String joinSql = SQLUtils.toMySqlString(join.getCondition());
                        if (LEFT_JOIN_PATTERN.matcher(joinSql).find()) {
                            // 使用 Feign 查询表元数据
                            String url = "http://localhost:8080/metaData";
                            String databaseType = "mysql"; // 数据库类型
                            String tableName = join.getRight().toString(); // 右表名
                            Feign.Builder builder = Feign.builder().encoder(new JacksonEncoder()).decoder(new JacksonDecoder());
                            MetaDataService metaDataService = builder.target(MetaDataService.class, url);
                            TableMetaData tableMetaData = metaDataService.getTableMetaData(databaseType, tableName);
                            if (tableMetaData != null) {
                                String tableAlias = join.getRight().getAlias();
                                if (tableAlias == null) {
                                    tableAlias = join.getRight().toString();
                                }
                                String primaryKey = tableMetaData.getPrimaryKey();
                                StringBuilder sb = new StringBuilder();
                                sb.append("select a.*, b.* from ");
                                sb.append(join.getLeft().toString());
                                sb.append(" a left join ");
                                sb.append(join.getRight().toString());
                                sb.append(" b on a.");
                                sb.append(primaryKey);
                                sb.append(" = b.");
                                sb.append(primaryKey);
                                sb.append(" where ");
                                sb.append(join.getLeft().toString());
                                sb.append(".");
                                sb.append(primaryKey);
                                sb.append(" is not null");
                                if (tableAlias != null) {
                                    sb.append(" and ");
                                    sb.append(tableAlias);
                                    sb.append(".");
                                    sb.append(primaryKey);
                                    sb.append(" is null");
                                }
                                queryBlock.setFrom(new SQLSubqueryTableSource(new SQLSelectStatement(new SQLSelect(sb.toString()), databaseType)));
                                join.setJoinType(SQLJoinTableSource.JoinType.INNER_JOIN);
                                join.setCondition(null);
                            }
                        }
                    }
                }
                return SQLUtils.toMySqlString(selectQuery);
            }
        }
        return sql;
    }
}

interface MetaDataService {
    @RequestLine("GET /metaData?databaseType={databaseType}&tableName={tableName}")
    TableMetaData getTableMetaData(@Param("databaseType") String databaseType, @Param("tableName") String tableName);
}

class TableMetaData {
    private String tableName;
    private List<String> columnNames;
    private String primaryKey;

    public String getTableName() {
        return tableName;
    }

    public void setTableName(String tableName) {
        this.tableName = tableName;
    }

    public List<String> getColumnNames() {
        return columnNames;
    }

    public void setColumnNames(List<String> columnNames) {
        this.columnNames = columnNames;
    }

    public String getPrimaryKey() {
        return primaryKey;
    }

    public void setPrimaryKey(String primaryKey) {
        this.primaryKey = primaryKey;
    }

    @Override
    public String toString() {
        return "TableMetaData [tableName=" + tableName + ", columnNames=" + columnNames + ", primaryKey=" + primaryKey + "]";
    }
}

上述代码使用 Druid 解析 SQL，替换其中的 left join，然后使用 Feign 查询表元数据。其中 `MetaDataService` 接口用于定义查询表元数据的 API，`TableMetaData` 类用于存储表元数据。请根据实际情况修改代码。