Addons to Az.Sql to query the table APIs from the portal

Az.Sql.Extensions.psm1

<#
.EXAMPLE
$db = Get-AzSqlDatabase -ResourceGroupName <ResourceGroupName> -ServerName <ServerName> -DatabaseName <DatabaseName>
Get-AzSqlDatabaseSchema -ResourceId $db.ResourceId
#>
function Get-AzSqlDatabaseSchema {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory = $true, ValueFromPipelineByPropertyName = $true)]
        [string]$ResourceId
    )
    $schemasApi = $ResourceId + "/schemas" + "?api-version=2018-06-01-preview"
    $schemasResult = Invoke-AzRest -Method GET -Path $schemasApi
    $schemasInfo = $schemasResult.Content | ConvertFrom-Json -Depth 100

    $schemasInfo.value | ForEach-Object { Write-Output $_ }  
}

function Get-AzSqlDatabaseTable {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory = $true, ValueFromPipelineByPropertyName = $true)]
        [string]$ResourceId,

        [Parameter(Mandatory = $false)]
        [string[]]$SchemaName,

        [Parameter(Mandatory = $false)]
        [string[]]$TableName
    )
    $schemas = $SchemaName ?? (Get-AzSqlDatabaseSchema -ResourceId $ResourceId).name
    
    foreach ($schema in $schemas) {
        $tableApi = $ResourceId + "/schemas/" + $schema + "/tables" + "?api-version=2018-06-01-preview"
        $tableResult = Invoke-AzRest -Method GET -Path $tableApi
        $tableInfo = $tableResult.Content | ConvertFrom-Json -Depth 100

        $result = $tableInfo.value
        if ($TableName) {
            $result = $result | Where-Object { $_ -in $TableName }
        }
        $result | ForEach-Object { Write-Output $_ }
    }
}

function Get-AzSqlDatabaseColumn {
    [CmdletBinding()]
    param (
        [Parameter(Mandatory = $true, ValueFromPipelineByPropertyName = $true)]
        [string]$ResourceId,

        [Parameter(Mandatory = $false)]
        [string[]]$SchemaName,

        [Parameter(Mandatory = $false)]
        [string[]]$TableName,

        [Parameter(Mandatory = $false)]
        [string[]]$ColumnName
    )
    # Get all the schemas and tables to work on if not specified
    $schemas = $SchemaName ?? (Get-AzSqlDatabaseSchema -ResourceId $ResourceId).name
    $tables = $TableName ?? (Get-AzSqlDatabaseTable -ResourceId $ResourceId).name

    # UI progress rules
    $totalSchemas = $schemas.Count
    $totalTables  = $tables.Count
    $shouldUseProgress =  $totalSchemas * $totalTables -ge 5

    # Loop the schemas
    for ($schemaIndex = 0; $schemaIndex -lt $totalSchemas; $schemaIndex++) {
        
        # Set the loop value and update progress if it is a large operation
        $schema = $schemas[$schemaIndex]
        if ($shouldUseProgress) {
            Write-Progress -Id 1 -Activity "Schema" -PercentComplete (($schemaIndex / $totalSchemas) * 100) -Status $schema
        }

        # Loop the tables
        for ($tableIndex = 0; $tableIndex -lt $totalTables; $tableIndex++) {

            # Set the loop value and update progress if it is a large operation
            $table = $tables[$tableIndex]
            if ($shouldUseProgress) {
                Write-Progress -Id 2 -Activity "Table" -PercentComplete (($tableIndex / $totalTables) * 100) -Status $table
            }

            # Actual work
            $columnApi = $ResourceId + "/schemas/" + $schema + "/tables/" + $table + "/columns" + "?api-version=2018-06-01-preview"
            $columnResult = Invoke-AzRest -Method GET -Path $columnApi
            $columnInfo = $columnResult.Content | ConvertFrom-Json -Depth 100
    
            $result = $columnInfo.value
            if ($ColumnName) {
                $result = $result | Where-Object { $_ -in $ColumnName }
            }
            $result | ForEach-Object { Write-Output $_ }
        }
    }
}

$schemaCompleter = {
    [OutputType([System.Management.Automation.CompletionResult])]
    param(
        [string] $CommandName,
        [string] $ParameterName,
        [string] $WordToComplete,
        [System.Management.Automation.Language.CommandAst] $CommandAst,
        [System.Collections.IDictionary] $FakeBoundParameters
    )
            
    $CompletionResults = [System.Collections.Generic.List[System.Management.Automation.CompletionResult]]::new()
    $__resourceId = $FakeBoundParameters["ResourceId"]
    if ($__resourceId) {
            (Get-AzSqlDatabaseSchema -ResourceId $__resourceId).name | Where-Object { 
            $_ -like "*$WordToComplete*" 
        } | ForEach-Object { $CompletionResults.Add($_) }
    }

    return $CompletionResults
}

$tableCompleter = {
    [OutputType([System.Management.Automation.CompletionResult])]
    param(
        [string] $CommandName,
        [string] $ParameterName,
        [string] $WordToComplete,
        [System.Management.Automation.Language.CommandAst] $CommandAst,
        [System.Collections.IDictionary] $FakeBoundParameters
    )
                
    $CompletionResults = [System.Collections.Generic.List[System.Management.Automation.CompletionResult]]::new()
    $__resourceId = $FakeBoundParameters["ResourceId"]
    $__schemaName = $FakeBoundParameters["SchemaName"]
    if ($__resourceId -and $__schemaName) {
            (Get-AzSqlDatabaseTable -ResourceId $__resourceId -SchemaName $__schemaName).name | Where-Object { 
            $_ -like "*$WordToComplete*" 
        } | ForEach-Object { $CompletionResults.Add($_) }
    }
    
    return $CompletionResults
}

Register-ArgumentCompleter `
    -CommandName Get-AzSqlDatabaseTable, Get-AzSqlDatabaseColumn `
    -ParameterName SchemaName `
    -ScriptBlock $schemaCompleter

Register-ArgumentCompleter `
    -CommandName Get-AzSqlDatabaseTable, Get-AzSqlDatabaseColumn `
    -ParameterName TableName `
    -ScriptBlock $TableCompleter